Add variant of stringWithContentsOfFile that produces an error

It turns out that the [NSString stringWithContentsOfFile:] family of
methods do not have a way of reporting why a file could not be read;
[NSString stringWithContentsOfFile:usedEncoding:error:] will only set
the error object if there was a problem with decoding.

So, we cook our own variant that tries to figure out why opening the
file failed.

User-visible effect: in the GTK+ port, if you try to open a file from
the "open recent" menu that no longer exists, you will get a more
helpful error message.

0 files changed, 0 insertions, 0 deletions