workaround for buggy lexers

It seems that some lexers can be buggy when used together with git blame.
This results in a 500 error when a user tries to open the blame page in some cases.
The workaround is pretty simple. Not only fall back to plaintext when the file content can't be guessed, but also when the lexer fails.

This closes #1521 and #1646

/cc @DouweM @JobV It would be awesome to have this in a patch release for 7.11 or even in the release. But that's probably to late.

See merge request !683