tools: improve the install-* meson targets.
Continue installing the list of files to install even if a failure occured. In particular, I encountered a problem where script-fu was failing to install because the installed binary was being executed (and its memory probably mmap-ed). Hence the copy failed with: > [Errno 26] Text file busy: '/my/prefix/lib64/gimp/2.99/plug-ins/script-fu/script-fu' Yet trying to reinstall plug-ins while GIMP is being run, hence script-fu is being executed, is a very common development trick (for me at least). So instead, when such an error occurs, I simply save it and output all the exceptions in the end (where these warnings are properly visible), instead of failing everything. Of course, ideally the subtarget should not even try to install script-fu as it is unchanged. This is why I changed shutil.copy() by shutil.copy2() to install metadata, which include last access timestamp. Maybe this could be used to decide whether the installed file is already the last one, thus not retry, though I am actually unsure if this is enough (on the other hand, the ultimate check of bit-to-bit comparison may obviously be too much/slow which is counter-productive), which is why I am not doing this change right now.