raise a sensible error when attachments are misconfigured

Conflicts:
	spec/fixtures/db/retina_rails.sqlite3
	spec/paperclip_spec.rb
clean up diff
don't overwrite additional data attributes
fix test
Revert "don't overwrite additional data attributes"

This reverts commit 1822c74d098f826ee312eda6825b5fcb915b5810.
extract into error class
simply; add documentation
fix docs
one more formatting fix
make it work
Merge branch 'raise_sensible_error'

Conflicts:
	lib/retina_rails/paperclip.rb
	spec/fixtures/db/retina_rails.sqlite3
	spec/paperclip_spec.rb