mtr should fail for warnings and missing result files

mtr should fail for warnings and missing result files

This is to ensure that one cannot push wrong test cases to protected branches

Note that while developing or testing it is common to temporary create a test case without a result file. Because of this, mtr should not give an error, only a warning, when running mtr on a single test cases. However when running a full test, any warnings or missing result files should result in an error.