Improve schema validation error messages

[Pennycook]

We previously decided to suppress jsonschema.exceptions.ValidationError messages because they provided a significant amount of context by default. By extracting the 'message' field, we can provide a helpful error message without also providing full context about the schema.

Related issues

Closes #67.

Proposed changes

• Include the 'message' field in the ValueError raised by a validation failure.
• Catch ValueError explicitly, since catching BaseException may hide other failures.
• Fix a small bug in the validation docstring.

---

Before this change, any error in the .cbi/config file produced this error:

error: Configuration file failed validation

Now, it produces helpful errors:

[compiler."c++"]
options = ["-DASDF", 1]
~
error: Failed schema validation against schema/cbiconfig.schema: 1 is not of type 'string'

[compiler."c++"]
options = "-DASDF"
~                   
error: Failed schema validation against schema/cbiconfig.schema: '-DASDF' is not of type 'array'