bugfix/improve error handling of specifying the grid

[twsearle]

Description

If a grid specification file is not found, orca-jedi will attempt to interpret the file path as a grid type. This leads to a confusing error when the name is not found.

This change checks for the validity of the grid option as a named ORCA grid first, if it is not found, then we assume the grid is a path to a specification file. If the file is not found then a more sensible error is raised.

Impact

No downstream impact.

Checklist

• I have updated the unit tests to cover the change
• New functions are documented briefly via Doxygen comments in the code
• I have linted my code using cpplint
• I have run the unit tests
• I have run mo-bundle to check integration with the rest of JEDI and run the unit tests under all environments

[s-good]

Looks good to me, thanks.