Improve error handling

[dholth]

Rever will try to substitute dicts into format strings, but will not print anything about the data that caused the error.

Instead, it should augment the builtin exceptions with a list of missing keys in the dict, or prompt the user for the missing value.

For example, use https://docs.python.org/3.1/library/string.html#string.Formatter.parse to show which $substitution in the string is missing.