Reintroduce early-failing language exceptions

[joeflack4]

Description

Fail early and fail often.
- Gandalf

Reintroduce:

class InvalidLanguageException(OdkFormException)
...
class AmbiguousLanguageError(InvalidLanguageException):
...
class InconsistentLabelLanguage(InvalidLanguageException):

The test files should be put under test/static/exceptions/language.

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Attachments

1. test-files.zip