schema: refactor validators and add pre-check for version

[albestro]

Close #247

Rationale:
Current version of stackinator does not support older versions of config.yaml (as stated by the error message reported below), so I don't see the point in having a back-compatible schema (like I was trying with #247).

stackinator/stackinator/recipe.py

Lines 58 to 67 in a4db2ee

	self._logger.error(
	"\nThe recipe is an old version 1 recipe for Spack v0.23 and earlier.\n"
	"This version of Stackinator supports Spack 1.0, and has deprecated support for Spack v0.23.\n"
	"Use version 5 of stackinator, which can be accessed via the releases/v5 branch:\n"
	" git switch releases/v5\n\n"
	"If this recipe is to be used with Spack 1.0, then please add the field 'version: 2' to\n"
	"config.yaml in your recipe.\n\n"
	"For more information: https://eth-cscs.github.io/stackinator/recipes/#configuration\n"
	)
	raise RuntimeError("incompatible uenv recipe version")

Proposal:
the schema should support just the current supported version, same for tests which should just care about currently supported version (at most, check that unsupported version are correctly reported with a user-friendly error message).

Changelog:

• move version check logic as a pre-check in the validation step
• adapt tests and test resources to version: 2
• took the chance to customize the error message (more in following comment)

[albestro]

About the error messages:

• it does not print anymore the schema. not very useful if not used to schemas, and it can easily be a wall of text hiding the actual problem.
• it reports all error messages and not just the first one. In the use-cases I faced, the first error is not always the most informative one to solve the problem.
• it prints the json path pointing to where the error is in the instance, opposed to the schema path
• it prints "ValidationError for {title}", where title is the one read from schema (I should probably change from for to in).

Note1: trivial examples verified against schema fixed as in #252

Example 1

name: prgenv-gnu
spack:
  repo: https://github.com/spack/spack.git
  commit: c702f5b89eea1950c76472fbf8f0919ad12880a6
  packages:
    repo: https://github.com/spack/spack-packages.git
store: /user-environment
description: GNU Compiler toolchain with cray-mpich, Python, CMake and other development tools.
version: 2

BEFORE

'commit' is a required property

Failed validating 'required' in schema['properties']['spack']['properties']['packages']:
    {'type': 'object',
     'additionalProperties': False,
     'required': ['repo', 'commit'],
     'properties': {'repo': {'type': 'string'},
                    'commit': {'oneOf': [{'type': 'string'},
                                         {'type': 'null'}],
                               'default': None}}}

On instance['spack']['packages']:
    {'repo': 'https://github.com/spack/spack-packages.git'}

NOW

ValidationError for 'Schema for Spack Stack config.yaml recipe file'
- Failed validating 'required' in $.spack.packages : 'commit' is a required property

Example 2

name: prgenv-gnu
spack:
  repo: https://github.com/spack/spack.git
  packages:
    repo: https://github.com/spack/spack-packages.git
store: /user-environment
description: GNU Compiler toolchain with cray-mpich, Python, CMake and other development tools.
version: 2

BEFORE

'commit' is a required property

Failed validating 'required' in schema['properties']['spack']:
    {'type': 'object',
     'additionalProperties': False,
     'required': ['repo', 'commit', 'packages'],
     'properties': {'repo': {'type': 'string'},
                    'commit': {'oneOf': [{'type': 'string'},
                                         {'type': 'null'}],
                               'default': None},
                    'packages': {'type': 'object',
                                 'additionalProperties': False,
                                 'required': ['repo', 'commit'],
                                 'properties': {'repo': {'type': 'string'},
                                                'commit': {'oneOf': [{'type': 'string'},
                                                                     {'type': 'null'}],
                                                           'default': None}}}}}

On instance['spack']:
    {'repo': 'https://github.com/spack/spack.git',
     'packages': {'repo': 'https://github.com/spack/spack-packages.git'}}

NOW

ValidationError for 'Schema for Spack Stack config.yaml recipe file'
- Failed validating 'required' in $.spack : 'commit' is a required property
- Failed validating 'required' in $.spack.packages : 'commit' is a required property

[bcumming]

Thanks @albestro. The new error messages for schema validation parsers are a big improvement to the usability of the tool.