added exception if environment variable doesn't match existing keys

.gitignore

@@ -3,3 +3,5 @@
 .coverage
 .eggs/
 *.egg-info/
+__pycache__/
+.cache/

fractalis/config.py

@@ -20,4 +20,8 @@ config = {
 
 def configure_app(app):
     mode = os.getenv('FRACTALIS_MODE', default='production')
-    app.config.from_object(config[mode])
+    try:
+        app.config.from_object(config[mode])
+    except KeyError as e:
+        raise KeyError("'{}' is no valid value for the FRACTALIS_MODE "
+                        "environment variable.".format(mode)) from e

tests/test_config.py

 import os
 import fractalis
+import pytest
 from importlib import reload
 
 class TestConfig:
@@ -27,3 +28,8 @@ class TestConfig:
         reload(fractalis)
         assert not fractalis.app.config['DEBUG']
         assert not fractalis.app.config['TESTING']
+
+    def test_config_when_unknown_mode(self):
+        os.environ['FRACTALIS_MODE'] = 'foobar'
+        with pytest.raises(KeyError):
+            reload(fractalis)