Abort early if rvm current doesn't return an rvm version

[cldwalker]

Motivation

RVM current may not return a valid RVM version, right now this causes a non-obvious failure in rvm_run.

Acceptance Criteria

• When rvm current is empty, abort early and notify the user what is wrong (rvm current is empty) and how to fix it.

[aredington]

No longer valid.