Bug 22194: Add Koha::Exceptions::Metadata

Bug 22194: Add Koha::Exceptions::Metadata

This trivial patch adds a basic Koha::Exceptions::Metadata exception and
Koha::Exceptions::Metadata::Invalid for using when the data cannot be
decoded (maybe because of incompatibility between format, schema, or
just bad data).

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/Koha/Exceptions.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>