[IMP] repo creation error handling + warnings

* The repo would only be registered at the very end of the creation,
  meaning an error *during* the repo creation (e.g. while uploading the
  first blob or setting up webhooks) would leave the repository
  undeleted. Register the repository as soon as we know it was
  created, in order to correctly dispose of it afterwards.
* Migrate logging.warning call to warnings.warn on repository deletion
  failure: pytest will print warnings during its reporting, not so for
  log warnings (?)

conftest.py

@@ -54,6 +54,7 @@ import subprocess
 import sys
 import time
 import uuid
+import warnings
 import xmlrpc.client
 from contextlib import closing
 
@@ -383,6 +384,7 @@ def make_repo(capsys, request, config, tunnel, users):
             'allow_rebase_merge': False,
         })
         r.raise_for_status()
+        repo = Repo(github, fullname, repos)
 
         # create webhook
         github.post('{}/hooks'.format(repo_url), json={
@@ -401,8 +403,9 @@ def make_repo(capsys, request, config, tunnel, users):
             'content': base64.b64encode(b'whee').decode('ascii'),
             'branch': 'garbage_%s' % uuid.uuid4()
         }).raise_for_status()
-
-        return Repo(github, fullname, repos)
+        # try to unwatch repo, doesn't actually work
+        repo.unsubscribe()
+        return repo
 
     yield repomaker
 
@@ -433,9 +436,6 @@ class Repo:
         self.hook = False
         repos.append(self)
 
-        # unwatch repo
-        self.unsubscribe()
-
     @property
     def owner(self):
         return self.name.split('/')[0]
@@ -470,7 +470,7 @@ class Repo:
     def delete(self):
         r = self._session.delete('https://api.github.com/repos/{}'.format(self.name))
         if r.status_code != 204:
-            logging.getLogger(__name__).warning("Unable to delete repository %s", self.name)
+            warnings.warn("Unable to delete repository %s (HTTP %s)" % (self.name, r.status_code))
 
     def set_secret(self, secret):
         assert self.hook