Audit panics

[szokeasaurusrex]

Audit the Sentry CLI codebase for panic! and other panicking functions/macros (e.g. expect, unwrap) to ensure that we should actually be panicking in these cases, rather than returning an error.

Several places in the codebase use panics for potentially recoverable errors; these should be updated to return errors instead. Sentry CLI should only panic if there is a bug, or if an unexpected state is encountered.