Alter the error message returned when peer selection fails

Alter the error message returned when peer selection fails

The Tahoe2PeerSelector returned either NoSharesError or NotEnoughSharesError
for a variety of error conditions that weren't informatively described by them.
This patch creates a new error, UploadHappinessError, replaces uses of
NoSharesError and NotEnoughSharesError with it, and alters the error message
raised with the errors to be more in line with the new servers_of_happiness
behavior. See ticket #834 for more information.