Improve error message when a script can't be run

[spli]

If the Processor service is functional but a script cannot be run, for instance due to an import error (e.g. numpy not installed) a 'No Processor' error is returned. This is misleading, instead we should distinguish between failure of the service (#8266) and failure of the script.

[wmoore]

What I have done in these cases is to wrap the import in try/except, then test for this when the script is run and return a suitable message to the user. For example, in my OMERO.figure PDF generating script

try:
    from reportlab.pdfgen import canvas
    from reportlab.lib.styles import getSampleStyleSheet
    from reportlab.lib.units import inch
    from reportlab.platypus import Paragraph, Frame
    # from reportlab.lib.pagesizes import letter, A4
    reportlabInstalled = True
except ImportError:
    reportlabInstalled = False

Then

        if not reportlabInstalled:
            client.setOutput(
                "Message",
                rstring("Install https://bitbucket.org/rptlab/reportlab"))
        else:
            // run the script

[jamoore]

A block like Will's makes sense where there's a special requirement (though, likely we should have a helper method to do this), but is the larger issue covered by #8555?

[spli]

#8555 covers import errors, I was envisaging a more general error handler.