codegen: surface batch validation errors

[chrisyxlee]

As a result of #1841 (letting named params contribute to param count), the call to ParamRef will actually fail for queries with mixed usage of numbered and named parameters.

This PR mainly ensures that when the failure occurs, the error surfaced to the user makes it obvious what the issue is. Previously, the error would say :batch* commands require parameters when the actual error is that the parameters are being mixed.

Now it says path/to/query.sql:1:1: could not determine data type of parameter $1 or
path/to/query.sql:15:1: can not mix $1 format with ? format

With this change, I believe mixed parameters are still allowed for non-batch queries, and I'm not sure if I should change that (please let me know through review).

#1625

[kyleconroy]

@chrisyxlee Can you include an example in the comments of a query that will produce the error? I want to make sure we add a test case for this.

[kyleconroy]

The validation code around parameters has changed a bit. ? place holders aren't allowed anymore in PostgreSQL queries, so I don't think this is needed anymore. If it is, please let me know.

[chrisyxlee]

Thank you! Apologies for forgetting about this PR -- and sounds good to me. 👍