Improve error messages/reporting from deft tracked compiler

[hansihe]

Right now only the happy case is handled, we want better diagnostics from the compiler.

• [x] Nicely format internal compiler errors
• [x] Report when stub functions are used in non-return positions
• [x] Tracing information collected in a better way
• [ ] When stub functions are used in non-return positions, give a reason why the compiler can't analyze it