Reframe prints "Unknown error." whenever the test fails in a phase other than sanity and performance

[ekouts]

You can see this error even when running the deps_complex.py test:

$ ./bin/reframe -c unittests/resources/checks_unlisted/deps_complex.py -r 

[ReFrame Setup]
  version:           3.2-dev1 (rev: f42347ef)
  command:           './bin/reframe -c unittests/resources/checks_unlisted/deps_complex.py -r'
...
  * Job type: local (id=31724)
  * Maintainers: []
  * Failing phase: sanity
  * Rerun with '-n T2 -p builtin --system generic:default'
  * Reason: sanity error: 31 != 30
Sanity check failure
------------------------------------------------------------------------------
FAILURE INFO for T7 
  * Test Description: T7
  * System partition: generic
  * Environment: None
  * Stage directory: None
  * Node list: None
  * Job type: batch job (id=None)
  * Maintainers: []
  * Failing phase: startup
  * Rerun with '-n T7 -p None --system generic'
  * Reason: task dependency error: dependencies failed
Unknown error.
------------------------------------------------------------------------------
....

The fix is very simple, it is caused by this line. I think the final message (Sanity check failure) is not useful, and I would suggest we omit it completely. We already have the * Failing phase: <phase> line in the report.