Improve an error message.

See #1397.
Show the error message only if the error actually occurred.
Improve an error message.

See #1362.
If we can't list sources, assume that the dependency hasn't changed.

Fixes #1397.
Tweak an error message.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	cabal-install/Distribution/Client/Install.hs
	cabal-install/Distribution/Client/Sandbox.hs