Move requirements checking earlier in the process

This allows us to check requirements before importing rest of the code,
so that we can tell user in nice way that something is broken instead of
ImportError of deep submodules.

Issue #427

weblate/__init__.py

@@ -86,3 +86,8 @@ def get_doc_url(page, anchor=''):
         url += '#%s' % anchor
 
     return url
+
+# Check for requirements
+from weblate.trans.requirements import check_requirements
+
+check_requirements()

weblate/trans/__init__.py

@@ -41,25 +41,3 @@ def create_permissions_compat(app, **kwargs):
         except AttributeError as error:
             # See https://code.djangoproject.com/ticket/20442
             print 'Failed to create permission objects: {0}'.format(error)
-
-
-@receiver(post_syncdb)
-@receiver(post_migrate)
-def check_versions(sender, app, **kwargs):
-    '''
-    Check required versions.
-    '''
-    if (app == 'trans'
-            or getattr(app, '__name__', '') == 'weblate.trans.models'):
-        from weblate.trans.requirements import get_versions, check_version
-        versions = get_versions()
-        failure = False
-
-        for version in versions:
-            failure |= check_version(*version)
-
-        if failure:
-            raise Exception(
-                'Some of required modules are missing or too old! '
-                'Check above output for details.'
-            )

weblate/trans/requirements.py

@@ -225,3 +225,20 @@ def get_versions_string():
             )
         )
     return '\n'.join(result)
+
+
+def check_requirements():
+    '''
+    Performs check on requirements and raises an exception on error.
+    '''
+    versions = get_versions()
+    failure = False
+
+    for version in versions:
+        failure |= check_version(*version)
+
+    if failure:
+        raise Exception(
+            'Some of required modules are missing or too old! '
+            'Check above output for details.'
+        )