qubes/tests: Show errors while loading external tests
qubes/tests: Show errors while loading external tests

Now failure to load external tests shows in which entry point the error
happened and a useful traceback. The traceback extends from the "try"
statement down to the actual error line, but it does not include the
frames above, ie. from the invocation to the load_tests routine. This is
a limitation of Python itself and usually not a problem.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin