Better error messages for hypothesis write <some_filename>

[Zac-HD]

The Ghostwriter CLI expects to be passed a module name (e.g. hypothesis write gzip), or dotted path to a callable (e.g. hypothesis write numpy.matmul). We also include the current directory in the python path, so that you can pass e.g. myscript.

If you try passing a filename instead, this doesn't work - whether myscript.py or path/to/myscript.py. I don't want to make that work, but we could have better error messages:

• hypothesis write myscript.py fails with Error: Found the 'script' module, but it doesn't have a 'py' attribute. In this case, we should add "\n Remember that the ghostwriter should be passed the name of a module, not a file"
• hypothesis write src/myscript.py fails with Error: Invalid value for 'FUNC...': my/script.py. We should add a similar explicit error message.