Detect wrong assert messages with empty string or with delta value.

Fixes issue https://github.com/Catrobat/Catroid/issues/150

Successful testrun http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test-source-and-jUnit-tests-only/72/

Changes:
The first parameter in the regex can't be an empty string anymore.
An additional regex which checks, if the assert message starts with all possible number prefixes (e.g. [0-9] | - | .)