Issue #11.  Use built-in sphinx errors and check for function existence.

This was my fix for Issue #11.  The builtin sphinx errors are used and the existence of the function is tested in the module.

Error output has this appearance, which includes source and line number

pickling environment... C:\Src\Git\nornir-buildmanager\nornir_buildmanager\build.py:docstring of nornir_buildmanager.build:5: ERROR: Module "nornir_buildmanager.build" has no attribute "BuildParserRootfs"
Incorrect argparse :module: or :func: values?
C:\Src\Git\nornir-buildmanager\nornir_buildmanager\build.py:docstring of nornir_buildmanager.build:5: ERROR: Module "nornir_buildmanager.build" has no attribute "BuildParserRootfs"
Incorrect argparse :module: or :func: values?
done