Runtime errors appear as Rust panics

[LightAndLight]

Fallible operations like subtraction (underflow) and division (divide by zero) make Rust panic. Ipso needs to handle these failures and report its own stack trace.

Without this, users have no way of knowing which part of their program caused the error.

Panics that should be errors-with-source-locations:

• Pattern match failures
• Arithmetic errors
• cmd.run failures