Do not use lyxerr in from_ascii(char const *)

Do not use lyxerr in from_ascii(char const *)

This function is used a lot to initialize static docstrings. This is
an issue in theory because lyxerr may not have been initialized in
time.

In practice of course, lyxerr will only be used when there is a
non-ascii character in the parameter, which is really not supposed to
happen.

This gets rid of 75 coverity reports.