Soften language on parse warnings

The "parse errors" are sometimes named "marker errors" (terminology which potx got rid of quite some time ago) and elsewhere called "parse errors". Actually, these are rather warnings and might not indicate actual errors in use of the APIs parsed, so I've softened the wording on the user interface to "source code warnings" instead and added help text which points out the fact that these might not be actual problems if the API is used properly elsewhere to catch the strings in question.

This is as part of an effort to step up l.d.o as a source code review / quality assurance helper.

Run tests and all seemed fine (although we do not have tests for warnings, nothing else broke). So committing right away.