#1994 - Make get_file_metadata throw an exception if photo or movie is unidentifiable/illegal.

- photo & movie helpers: modified to throw exceptions when file is known to be unidentifiable/illegal.
- item model: revised to work with exceptions and be more explicit when the data file is invalid.
- item model: removed duplicate get_file_metadata call for updated items.
- admin_watermarks controller: revised to work with exceptions (really cleans up logic here).
- graphics helper: revised to handle invalid placeholders (a nearly-impossible corner case, but still...).
- photo & movie helper tests: revised to work with exceptions, added new tests for illegal files with valid extensions.
- item model tests: revised to work with exceptions, added new tests for illegal files with valid extensions.