If mkdocs.yml is broken raise a specific exception and catch it properly

[humitos]

Currently, we are raising BuildEnvironmentError which is too generic and produce the error to be logged into sentry when it's not an app bug but a user configuration problem.

https://github.com/rtfd/readthedocs.org/blob/df85fefc5f59c7a00bcc1ffe23965efe75e8c895/readthedocs/doc_builder/backends/mkdocs.py#L84-L87

We should raise something like MkDocsYAMLError (or a better name) and add it as a warning exception at,

https://github.com/rtfd/readthedocs.org/blob/df85fefc5f59c7a00bcc1ffe23965efe75e8c895/readthedocs/doc_builder/environments.py#L433-L441