Explicit error for wrong resource metadata format

When creating sample with wrong resource metadata format, Ceilometer CLI
will return a ValueError about json decoding, this change provide a more
explicit error info.

Change-Id: I18e757ddb319fbf0ffad13c07528355785ea5e9a
Closes-Bug: 1465193

ceilometerclient/tests/unit/v2/test_shell.py

@@ -576,6 +576,16 @@ class ShellSampleCreateCommandTest(utils.BaseTestCase):
 +-------------------+---------------------------------------------+
 ''', sys.stdout.getvalue())
 
+    def test_sample_create_with_invalid_resource_metadata(self):
+        self.args.resource_metadata = 'foo=bar'
+        with mock.patch('ceilometerclient.exc.CommandError') as e:
+            e.return_value = exc.BaseException()
+            self.assertRaises(exc.BaseException,
+                              ceilometer_shell.do_sample_create,
+                              self.cc, self.args)
+            e.assert_called_with('Invalid resource metadata, it should be a'
+                                 ' json string, like: \'{"foo":"bar"}\'')
+
 
 class ShellSampleCreateListCommandTest(utils.BaseTestCase):
 

ceilometerclient/v2/shell.py

@@ -231,7 +231,12 @@ def do_sample_create(cc, args={}):
         k, v = var[0], var[1]
         if v is not None:
             if k == 'resource_metadata':
-                fields[k] = json.loads(v)
+                try:
+                    fields[k] = json.loads(v)
+                except ValueError:
+                    msg = ('Invalid resource metadata, it should be a json'
+                           ' string, like: \'{"foo":"bar"}\'')
+                    raise exc.CommandError(msg)
             else:
                 fields[arg_to_field_mapping.get(k, k)] = v
     sample = cc.samples.create(**fields)