machines: Don't swallow promise failures

This makes it possible to use the promises returned by the asynchronous
actions.

Closes #7121
This uses 'fail' instead of 'catch'.  Also, it intercepts the
getVirtProvider errors only and not errors from the actual method call
as well.

pkg/machines/libvirt.es6

@@ -265,7 +265,7 @@ function spawnVirsh({connectionName, method, failHandler, args}) {
         cmd: 'virsh',
         args: VMS_CONFIG.Virsh.connections[connectionName].params.concat(args),
         failHandler,
-    }).catch((ex, data, output) => {
+    }).fail((ex, data, output) => {
         const msg = `${method}() exception: '${ex}', data: '${data}', output: '${output}'`;
         if (failHandler) {
             logDebug(msg);

pkg/machines/provider.es6

@@ -111,15 +111,17 @@ function getVirtProvider (store) {
  * Lazily initializes the virt provider and dispatches given method on it.
  */
 export function virt(method, action) {
-    return (dispatch, getState) => getVirtProvider({dispatch, getState}).then(provider => {
+    return (dispatch, getState) => getVirtProvider({dispatch, getState}).fail(err => {
+        console.error('could not detect any virt provider');
+    }).then(provider => {
         if (method in provider) {
             logDebug(`Calling ${provider.name}.${method}`, action);
             return dispatch(provider[method](action));
         } else {
-            console.warn(`method: '${method}' is not supported by provider: '${provider.name}'`);
+            var msg = `method: '${method}' is not supported by provider: '${provider.name}'`;
+            console.warn(msg);
+            return cockpit.reject(msg);
         }
-    }).catch(err => {
-        console.error('could not detect any virt provider');
     });
 }