Warning when user doesn't provide a file

[paulkorir]

Something better than:

~$ sff-migrate
Traceback (most recent call last):
  File "/Users/pkorir/Envs/sfftk-python3.7/bin/sff-migrate", line 10, in <module>
    sys.exit(main())
  File "/Users/pkorir/Envs/sfftk-python3.7/lib/python3.7/site-packages/sfftk_migrate/main.py", line 83, in main
    status = do_migration(args)
  File "/Users/pkorir/Envs/sfftk-python3.7/lib/python3.7/site-packages/sfftk_migrate/migrate.py", line 85, in do_migration
    source_version = get_source_version(args.infile)
  File "/Users/pkorir/Envs/sfftk-python3.7/lib/python3.7/site-packages/sfftk_migrate/core.py", line 107, in get_source_version
    source_tree = etree.parse(fn)
  File "src/lxml/etree.pyx", line 3519, in lxml.etree.parse
  File "src/lxml/parser.pxi", line 1839, in lxml.etree._parseDocument
  File "src/lxml/parser.pxi", line 1865, in lxml.etree._parseDocumentFromURL
  File "src/lxml/parser.pxi", line 1769, in lxml.etree._parseDocFromFile
  File "src/lxml/parser.pxi", line 1163, in lxml.etree._BaseParser._parseDocFromFile
  File "src/lxml/parser.pxi", line 601, in lxml.etree._ParserContext._handleParseResultDoc
  File "src/lxml/parser.pxi", line 711, in lxml.etree._handleParseResult
  File "src/lxml/parser.pxi", line 638, in lxml.etree._raiseParseError
OSError: Error reading file '': failed to load external entity ""