Better error handling.

[pchampin]

The error messages often contain the location of the error. This location information should be extracted and presented in a more uniform and readable way.

If/when #4 is implemented, it would even be better to highlight the error directly in the input editor.

[pchampin]

#9 partially addresses this issue, by

• parsing the text of the exception (for syntaxes of the "turtle-family"), then
• highlighting the correct place in the monaco-editor, with the corresponding message

I keep this issue open because a more robust solution (also working for the other parsers) would be preferable.