SCM/CC: Internationalize all warning and error messages

The CG says that all messages should be internationalized, except

• ‘programming_error ()’s
• ‘programming_warning ()’s
• debug strings
• output strings (PostScript, TeX, etc.)

Currently this is only partly the case, e.g. in SCM:

git grep '(ly:warning (_' scm | wc -l
54
git grep '(ly:warning "' scm | wc -l
25 

Therefore add the appropriate gettext shortcuts in SCM and C++, whereever possible.