system-test: downgrade missing trust from an error to a warning

Currently, the system-test module checks whether the environment is
properly configured to run secure clients against the system-test.
If this test fails then the system-test build fails.

There are (at least) a couple of reasons why failing the build is
wrong:

  1.  there are situations where building the system-test should not
      dependend on being able to successfully test dCache via
      system-test (e.g., "mvn release")

  2.  the user may use a client that is configured to use the
      trust-store provided within system-test.  Therefore, missing
      trust in the default locations isn't always a problem.

This patch downgrades the test failing from an error to a warning.  The
patch also rewords the message to try to avoid some confusion
experienced by users.

Patch: https://rb.dcache.org/r/5533/
Acked-by; Tigran Mkrtchyan
Target: master