Improve error message when PLC project name misspelled

[klauer]

Vincent found that having his PLC project named incorrectly meant that the startup script didn't get created and this weird exception was thrown:

Traceback (most recent call last):
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/bin/pytmc", line 10, in <module>
    sys.exit(main())
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/pytmc/bin/pytmc.py", line 91, in main
    func(**kwargs)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/pytmc/bin/template.py", line 742, in main
    raise stashed_exception
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/pytmc/bin/template.py", line 735, in main
    rendered = render_template(template_text, template_args)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/pytmc/bin/template.py", line 621, in render_template
    return env.get_template('template').render(context)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "<template>", line 8, in top-level template code
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.6.0/lib/python3.9/site-packages/jinja2/environment.py", line 474, in getattr
    return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'None' has no attribute 'filename'
make[1]: *** [st.cmd] Error 1
make[1]: Leaving directory `/cds/group/pcds/epics-dev/espov/ioc/mfx/lcls-plc-mfx-motion/iocBoot/ioc-mfx-motion'
make: *** [build] Error 2

This could be improved in the template by checking for a None plc and reporting an error/exiting somehow