Better error message when org doesn't exist

* Produce a better error message when the destination org doesn't exist.
* Support pushing to the authenticated user's account

**Before**
`error creating github repository `synced-actions/setup-node`: error creating repository: POST http://my-ghes-hostname/api/v3/orgs/synced-actions/repos: 404 Not Found []`

**After**
`error creating github repository `synced-actions/setup-node`: Organization `synced-actions` doesn't exist at http://my-ghes-hostname. You must create it first.`

https://github.com/actions/actions-sync/issues/1

README.md

@@ -41,9 +41,9 @@ When there are machines which have access to both the public internet and the GH
 
 ```
   actions-sync sync \
-    --cache-dir "tmp/cache" \
+    --cache-dir "/tmp/cache" \
     --destination-token "token" \
-    --destination-url "www.example.com" \
+    --destination-url "https://www.example.com" \
     --repo-name actions/setup-node
 ```
 
@@ -97,7 +97,7 @@ When no machine has access to both the public internet and the GHES instance:
   bin/actions-sync push \
     --cache-dir "/tmp/cache" \
     --destination-token "token" \
-    --destination-url "http://www.example.com"
+    --destination-url "https://www.example.com"
 ```