libraries\Image::get_exif: Fix undefined index error

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Improve backtraces

PHP's default functions truncate longer argument values so backtrace
will not be as helpful as they could be. This code tries to mimic PHP's
way of printing traces except it prints full arguments.

Signed-off-by: Florian Pritz <bluewind@xinu.at>