ref(notifications): improve github check messages for PR status (#72)

CHANGES
- add message when PR is good to merge
- reduce some duplicated logic in mergeability for set_status

BEFORE
<img width="315" alt="Screen Shot 2019-07-17 at 7 22 52 PM" src="https://user-images.githubusercontent.com/1929960/61418371-4d944900-a8c8-11e9-80ca-b610b89bda10.png">
AFTER
<img width="307" alt="Screen Shot 2019-07-17 at 7 21 24 PM" src="https://user-images.githubusercontent.com/1929960/61418372-4d944900-a8c8-11e9-9fb3-336b838df0a4.png">

kodiak/evaluation.py

@@ -63,14 +63,23 @@ class MissingAppID(BaseException):
     another instance of kodiak.
     """
 
+    def __str__(self) -> str:
+        return "missing Github app id"
+
 
 class BranchMerged(BaseException):
     """branch has already been merged"""
 
+    def __str__(self) -> str:
+        return str(self.__doc__)
+
 
 class MergeConflict(BaseException):
     """Merge conflict in the PR."""
 
+    def __str__(self) -> str:
+        return "merge conflict"
+
 
 def review_status(reviews: typing.List[PRReview]) -> PRReviewState:
     """

kodiak/pull_request.py

@@ -150,16 +150,24 @@ async def mergeability(
                 valid_merge_methods=self.event.valid_merge_methods,
             )
             self.log.info("okay")
+            await self.set_status(summary="⛴ ready to merge")
             return MergeabilityResponse.OK, self.event
-        except MissingAppID:
-            return MergeabilityResponse.NOT_MERGEABLE, self.event
-        except NotQueueable as e:
-            await self.set_status(summary="🛑 cannot merge", detail=str(e))
-            return MergeabilityResponse.NOT_MERGEABLE, self.event
-        except MergeConflict:
-            await self.set_status(summary="🛑 cannot merge", detail="merge conflict")
-            if self.event.config.merge.notify_on_conflict:
+        except (NotQueueable, MissingAppID, MergeConflict, BranchMerged) as e:
+            if (
+                isinstance(e, MergeConflict)
+                and self.event.config.merge.notify_on_conflict
+            ):
                 await self.notify_pr_creator()
+
+            if (
+                isinstance(e, BranchMerged)
+                and self.event.config.merge.delete_branch_on_merge
+            ):
+                await self.client.delete_branch(
+                    branch=self.event.pull_request.headRefName
+                )
+
+            await self.set_status(summary="🛑 cannot merge", detail=str(e))
             return MergeabilityResponse.NOT_MERGEABLE, self.event
         except MissingGithubMergeabilityState:
             self.log.info("missing mergeability state, need refresh")
@@ -170,15 +178,6 @@ async def mergeability(
         except NeedsBranchUpdate:
             await self.set_status(summary="⏭ need update")
             return MergeabilityResponse.NEEDS_UPDATE, self.event
-        except BranchMerged:
-            await self.set_status(
-                summary="🛑 cannot merge", detail="branch merged already"
-            )
-            if self.event.config.merge.delete_branch_on_merge:
-                await self.client.delete_branch(
-                    branch=self.event.pull_request.headRefName
-                )
-            return MergeabilityResponse.NOT_MERGEABLE, self.event
 
     async def update(self) -> None:
         self.log.info("update")