Error should be clearer when .schema.json header cannot be found

[ethall]

Subject of the issue

When attempting to archive a file that does not contain a header expected by .schema.json, an IndexError is raised with a message saying that said header could not be found. However, it's not immediately clear that the column named in the IndexError is required because of its inclusion in the .schema.json file.

Example error:

Traceback (most recent call last):
  File "C:\Python36\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Python36\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Source\forked\syphon\syphon\__main__.py", line 99, in <module>
    bootstrap(argv)
  File "C:\Source\forked\syphon\syphon\__main__.py", line 15, in bootstrap
    exit(_main(args))
  File "C:\Source\forked\syphon\syphon\__main__.py", line 84, in _main
    archive(this_context)
  File "C:\Source\forked\syphon\syphon\archive\archive.py", line 113, in archive
    check_columns(context.schema, data_frame)
  File "C:\Source\forked\syphon\syphon\schema\checkcolumns.py", line 27, in check_columns
    'Cannot find column named "{}"'.format(header))
IndexError: Cannot find column named "blah"

Your environment

• Python 3.6.6
• Syphon 0.1.1

Steps to reproduce

Attempt to archive a file or files that do not contain one or more columns required by the .schema.json in the destination.

Expected behaviour

Error message should mention why the column is required.

Actual behaviour

Error just says that the column could not be found.