Aborts `NewAuthorization` when SA RPC fails.

The RA performs an RPC to the SA's `GetValidAuthorizations` function
when attempting to find existing valid authorizations to reuse. Prior to
this commit, ff the RPC fails (e.g. due to a timeout) the calling code
logs the failure as a warning but fails to return the error and cease
processing. This results in a nil panic when we later try to index
`auths`

This commit inserts the missing `return` to ensure we don't process
further, thereby resolving #2274.

ra/ra.go

@@ -373,6 +373,7 @@ func (ra *RegistrationAuthorityImpl) NewAuthorization(ctx context.Context, reque
 				fmt.Sprintf("unable to get existing validations for regID: %d, identifier: %s",
 					regID, identifier.Value))
 			ra.log.Warning(string(outErr))
+			return authz, outErr
 		}
 
 		if existingAuthz, ok := auths[identifier.Value]; ok {