Remove panic() and handle it appropriately (#5807)

This is an effort to remove panic from the source. 
Add a new call called CriticialIf, that calls LogIf and exits. 
Replace panics with one of CriticalIf, FatalIf and a return of error.

cmd/web-handlers.go

@@ -406,7 +406,8 @@ func (web webAPIHandlers) GenerateAuth(r *http.Request, args *WebGenericArgs, re
 	if !isHTTPRequestValid(r) {
 		return toJSONError(errAuthentication)
 	}
-	cred := auth.MustGetNewCredentials()
+	cred, err := auth.GetNewCredentials()
+	logger.CriticalIf(context.Background(), err)
 	reply.AccessKey = cred.AccessKey
 	reply.SecretKey = cred.SecretKey
 	reply.UIVersion = browser.UIVersion