[epm] Consider using stderr for debug, info, warn and error functions

[ilyash-b]

https://github.com/elves/elvish/blob/c55df583d56fafee790a186eb8d390a80148d1c1/eval/bundled/epm.elv.go#L33-L53

[zzamboni]

Good idea, should be easy to fix.

[krader1961]

FWIW, I think something like this should be implemented by a trace module that other modules can use. Both so the user has control of the output and can easily control whether a stack trace is included. This is related to issues #1183 and #803.