Handle invalid entries in .hgsubstate better and inform users (BB-5939)

If there is an invalid entry in the .hgsubstate file, the Bitbucket site may not show the source view and instead show an error. Fixing the file will fix the issue, but the site should handle this error better and inform the user of the invalid entry.

Specifically, the error returned by Mercurial is something like
{{{
abort: invalid subrepository revision specifier in .hgsubstate line xx
}}}
which can also be seen when cloning the repository.