Differentiate Go panics from other exit code 2 usage

[paultyng]

Exit code 2 is also used for Go panics, so in Plan we need to properly differentiate between a core panic and exit for the detailed exit code. Unsure how to simulate panics though, maybe we should go back through fixed ones to see if we can simulate one in a shipped release for testing.

[radeksimko]

Related to #158

I believe that custom error types which would include stderr might address this.