Added default error message to the Error object.
The message can be set from other applications
Merge pull request #1 from sumup/features/error_messages

Added default error message to the Error object. - passed review
make callback to handle errors if callbacks are defined
typo: remove duplicate calls to parse error
Set the version to 0.0.15.1
add rebuild script
new build of rack-rpc-sumup
Merge branch 'master' of github.com:sumup/rack-rpc

Conflicts:
	VERSION
Merge branch 'features/experimental'

Conflicts:
	rebuild.sh