getAllDevices broken by cameraMicroEnable4.py

While making pyCameras independent from imrpy I noticed, that getAllDevices() from utils is broken for me. While trying to scan for cameras implemented by cameraMicroEnable4.py I get a KeyError because on my system the environment variable SISODIR5 does not exist:

Traceback (most recent call last):
  File "/home/kroeger/kroeger/Code/pyCameras/pyCameras/utils.py", line 317, in <module>
    print(getAllDevices())
  File "/home/kroeger/kroeger/Code/pyCameras/pyCameras/utils.py", line 88, in getAllDevices
    implementation in listCameraImplementations()}
  File "/home/kroeger/kroeger/Code/pyCameras/pyCameras/utils.py", line 64, in listCameraImplementations
    classes.append(dynamic_import_class(modname + '.Camera'))
  File "/home/kroeger/kroeger/Code/pyCameras/pyCameras/utils.py", line 44, in dynamic_import_class
    mod = __import__(mod, fromlist=[cls])
  File "/home/kroeger/kroeger/Code/pyCameras/pyCameras/cameraMicroEnable4.py", line 25, in <module>
    sys.path.append(os.path.join(os.environ['SISODIR5'],
  File "/usr/lib/python3.5/os.py", line 725, in __getitem__
    raise KeyError(key) from None
KeyError: 'SISODIR5'

My suggestion would be to simply add a KeyError catch to line 31 in cameraMicroEnable4.py since not having the SISODIR5 environment variable would ultimately lead to being unable to import the necessary library.

I will go ahead and create a merge request for this. @melchert I think you wrote this file so I will mark you for that request...