Catch when Souffle "helpfully" continues with malformed input in tests.

[markww]

Souffle, in its attempt to be helpful, tries to keep going when it sees slightly malformed input. This is not what we want for tests, as it could causes spurious passing results (example issue with malformed input error message: #655 ). To fix this, we should augment our test recipes to catch malformed inputs. This could happen, for instance, by redirecting the error output of Souffle to a file and greping that file for the malformed error message output.