adding test and more informative error

repo2docker/app.py

@@ -177,7 +177,8 @@ class Repo2Docker(Application):
 
         if ref:
             if len(ref.split('/')) != 2:
-                raise ValueError('--ref must be of the form remote/ref')
+                raise ValueError('Expected --ref to be of the form '
+                                 'remote/reference, but got %s' % ref)
             try:
                 for line in execute_cmd(['git', 'reset', '--hard', ref],
                                         cwd=checkout_path,
@@ -247,8 +248,8 @@ class Repo2Docker(Application):
 
         argparser.add_argument(
             '--ref',
-            help=('If building a git url, which ref to check out. Must be of'
-                  'the form `remote/ref`. E.g., `origin/master`.')
+            help=('If building a git url, which reference to check out. Must '
+                  'be of the form `remote/reference`. E.g., `origin/master`.')
         )
 
         argparser.add_argument(

tests/argumentvalidation.py

@@ -138,7 +138,7 @@ def test_volume_no_run_fail():
 
 def test_env_no_run_fail():
     """
-    Test to check if repo2docker fails when both --no-run and -e arguments are given 
+    Test to check if repo2docker fails when both --no-run and -e arguments are given
     """
     builddir = os.path.dirname(__file__)
     args_list = ['--no-run', '-e', 'FOO=bar', '--']
@@ -231,3 +231,12 @@ def test_docker_no_build_success():
 
     assert validate_arguments(builddir, args_list, "", True)
 
+
+def test_ref_has_correct_form():
+    """
+    Test to check if --ref is given with the form `remote/reference`
+    """
+    builddir = os.path.dirname(__file__)
+    args_list = ['--no-run', '--no-build', '--ref', 'myreference']
+
+    assert not validate_arguments(builddir, args_list, 'Expected --ref to be of the form remote/reference, but got myreference')