Assert that JSVM source files in the INFO files exist #28

This was originally filed as #800, which was fixed in #1812. Unfortunately the assertion failed, so I made it a warning in #1819.
I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1522304 to track the LCOV rewriter bug. Once it is fixed, we'll be able to turn the warning back into an assertion.