On invalid LESS, throw exception rather than emit blanks

This patch makes lessphp throw errors when it is called to compile LESS code
that references undefined variables or mixins. A mixin invocation will cause an
exception to be thrown if it fails to match, whether it is because no mixin of
that name exists, or because the invocation fails to match against the
signatures or guards of defined mixins. This is consistent with the behavior of
the reference LESS implementation.

The patch removes cases from tests/inputs/* that fail to compile. A new test
suite, ErrorHandlingTest.php, verifies the new error-throwing behavior.