Rename "sender" to "value"

There's no reason it has to mirror AppKit/UIKit-style event handling.
General-purpose arguments can be valuable too.
Add RACCommand.errors for handling all errors generically
Fix documentation typo
Rewrite the -catch: as a -doError:

Also check self against nil before accessing the ivar. Not strictly
sure if this is necessary (can't find any docs on it), but it couldn't
hurt.
Test that nothing but errors are sent on the "errors" signal
Merge pull request #351 from ReactiveCocoa/command-errors

Add an errors signal to RACCommand