fio: help noticing fio package is missing

[sandrobonazzola]

Changes introduced with this PR

Fixes #42 by providing a more useful error when fio package is missing:

$ python test_fio_plugin.py 
Error: missing fio executable, please install fio package
F.
======================================================================
FAIL: test_functional_success (__main__.FioPluginTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "~/arcaflow-plugins/python/fio/test_fio_plugin.py", line 50, in test_functional_success
    self.assertEqual("success", output_id)
AssertionError: 'success' != 'error'
- success
+ error

----------------------------------------------------------------------
Ran 2 tests in 0.010s

FAILED (failures=1)

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Signed-off-by: Sandro Bonazzola sbonazzo@redhat.com