Display a decent error message when aborting due to FrozenFlaskWarning

Related #26 (comment)

elsa/_cli.py

@@ -1,4 +1,5 @@
 import os
+import sys
 import urllib.parse
 import warnings
 
@@ -39,7 +40,11 @@ def freeze_app(app, freezer, path, base_url):
     # make sure Frozen Flask warnings are treated as errors
     warnings.filterwarnings('error', category=flask_frozen.FrozenFlaskWarning)
 
-    freezer.freeze()
+    try:
+        freezer.freeze()
+    except flask_frozen.FrozenFlaskWarning as w:
+        print('Error:', w, file=sys.stderr)
+        sys.exit(1)
 
 
 def inject_cname(app):

tests/test_commands.py

@@ -291,6 +291,15 @@ def test_freeze_different_warning_is_fine(elsa):
     # tests just success of the command
 
 
+def test_freeze_mishmash_decent_error_msg(elsa, capsys):
+    elsa.run('freeze', script='mishmash.py', should_fail=True)
+    out, err = capsys.readouterr()
+    print('OUT', out)
+    print('ERR', err)
+    assert 'Traceback' not in err
+    assert 'does not match' in err
+
+
 def test_freeze_cname(elsa):
     elsa.run('freeze')
     with open(CNAME_FIXTURES) as f: