find file content using dynamic storage adapters in FileMetadata
find file content using dynamic storage adapters in FileMetadata

`FileMetadata#file` should work for all storage adapters. it previously assumed
that the storage adapter would be the configured one, but if a `FileMetadata`
exists with a `file_identifier` in another URI scheme, it should resolve rather
than error.

rescue the AdapterNotFound case because the prior behavior was to always
return a file, even if `file_identifier` is `nil`. for compatibility reasons, i
don't want to change that at least until a sharper interface with error handling
can be dreamed up.

samvera/hyrax,samvera/hyrax,samvera/hyrax,samvera/hyrax