Better exception error messages for Scala cells

[okennedy]

What pain point is this feature intended to address? Please describe.
Error reporting in scala cells sucks. Parse errors are incredibly uninformative, and when exceptions are reported, it's not entirely clear which parts of the code are responsible.

Describe the solution you'd like

• When a parse error occurs, we should point at the specific part of the code that failed to parse
• When an exception occurs, we should trim the stack trace to exclude 'internal' parts of the code.
• When an exception occurs, we should point at the specific part of the code where the exception occurred (e.g., see https://github.com/laech/java-stacksrc )

Describe alternatives you've considered
Continuing to suffer through uninformative exceptions.