[bugfix] Treat empty source code context when reporting errors

[synscc]

When a modulecmd python load fails with a module file, the frame's code context being None causes reframe to terminate prematurely.

SUMMARY OF FAILURES
/home/chungsy/projects/reframe.git/bin/reframe: run session stopped: type error: can only join an iterable
/home/chungsy/projects/reframe.git/bin/reframe: Traceback (most recent call last):
  File "/home/chungsy/projects/reframe.git/reframe/frontend/cli.py", line 1210, in main
    runner.stats.print_failure_report(printer)
  File "/home/chungsy/projects/reframe.git/reframe/frontend/statistics.py", line 210, in print_failure_report
    run_report = self.json()[-1]
  File "/home/chungsy/projects/reframe.git/reframe/frontend/statistics.py", line 164, in json
    entry['fail_reason'] = errors.what(*t.exc_info)
  File "/home/chungsy/projects/reframe.git/reframe/core/exceptions.py", line 370, in what
    source = ''.join(frame.code_context)
TypeError: can only join an iterable

After this patch, it's able to continue the normal error reporting process.

FAILURE INFO for GromacsTest_0
  * Expanded name: GromacsTest %pi=0
  * Description: GromacsTest %pi=0
  * System partition: cex:vanilla-mpi
  * Environment: builtin
  * Stage directory: /home/chungsy/projects/nscc-reframe-tests.git/stage/cex/vanilla-mpi/builtin/GromacsTest_0
  * Node list:
  * Job type: batch job (id=None)
  * Dependencies (conceptual): []
  * Dependencies (actual): []
  * Maintainers: []
  * Failing phase: run
  * Rerun with '-n GromacsTest_0 -p builtin --system cex:vanilla-mpi -r'
  * Reason: name error: <string>:32: name 'source' is not defined

The 'source' is actually a BASH source command included in a module file.

[jenkins-cscs]

Can I test this patch?

[vkarak]

I have just a minor suggestion.