unittest speed-up: construct error message only when assertion fails
unittest speed-up: construct error message only when assertion fails

dagophil/vigra,dstoe/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dstoe/vigra,dstoe/vigra