Docs nit: What to do for a failed/aborted install run

[jkopczyn]

I have had problems installing RVM and Ruby today; I have concluded that RVM was probably always fine and only Ruby malfunctioning, but it would have been better if RVM docs made clear what problems could or couldn't be created by interrupting the \curl and script run.

Based on my reading of the install docs, rerunning the download and script is safe (won't make things worse) and should repair any initial problems. (Also it may be written well enough that it shouldn't cause problems even if interrupted; it seems very well-maintained, so this seems plausible, but it remains a bash script, so not that plausible.) Adding an entry to the install page seems probably sufficient to edify/reassure other new users.