flaskr: report app init error instead of crashing

Improve error reporting of the application factory built during the tutorial
by not swallowing all OSError exceptions that could happen when trying to
create the instance folder.

docs/tutorial/factory.rst

@@ -35,6 +35,7 @@ directory should be treated as a package.
 .. code-block:: python
     :caption: ``flaskr/__init__.py``
 
+    import errno
     import os
 
     from flask import Flask
@@ -58,8 +59,9 @@ directory should be treated as a package.
         # ensure the instance folder exists
         try:
             os.makedirs(app.instance_path)
-        except OSError:
-            pass
+        except OSError as e:
+            if e.errno != errno.EEXIST:
+                raise
 
         # a simple page that says hello
         @app.route('/hello')

examples/tutorial/flaskr/__init__.py

@@ -1,3 +1,4 @@
+import errno
 import os
 
 from flask import Flask
@@ -23,8 +24,9 @@ def create_app(test_config=None):
     # ensure the instance folder exists
     try:
         os.makedirs(app.instance_path)
-    except OSError:
-        pass
+    except OSError as e:
+        if e.errno != errno.EEXIST:
+            raise
 
     @app.route('/hello')
     def hello():