adds line numbers to syntax errors

[c-brenn]

• Web.Error now takes the metadata as an argument
• Parser now adds metadata to the errors it generates - %{line: line_number}

[c-brenn]

Had a brief look at adding highlighting for syntax errors to the front end - a bit complicated with the given request chain/pml fetching setup.

I think we need to revisit app.js and do some refactoring.