Throw exeption when output can not be stored

If we do not do this, in case something goes wrong while storing the
output we get a response document that says process succeeded but has an
empty result.
Removed some warnings
Fixed a bug in ExecuteResponsbuilder, added tests

This should fix the bug Matthias Hinz has described about wrong mime
types beeing assigned to output data.
Merge branch 'executeresponsebuilder-bug'