Show friendly message to user but still log detail

Author: jcansdale

src/GitHub.App/Services/PullRequestService.cs

@@ -308,14 +308,21 @@ public IObservable<string> ExtractFile(
                 }
                 else
                 {
-                    sha = await gitClient.GetPullRequestMergeBase(
-                        repo,
-                        pullRequest.Base.RepositoryCloneUrl,
-                        pullRequest.Head.RepositoryCloneUrl,
-                        pullRequest.Base.Sha,
-                        pullRequest.Head.Sha,
-                        pullRequest.Base.Ref,
-                        pullRequest.Head.Ref);
+                    try
+                    {
+                        sha = await gitClient.GetPullRequestMergeBase(
+                            repo,
+                            pullRequest.Base.RepositoryCloneUrl,
+                            pullRequest.Head.RepositoryCloneUrl,
+                            pullRequest.Base.Sha,
+                            pullRequest.Head.Sha,
+                            pullRequest.Base.Ref,
+                            pullRequest.Head.Ref);
+                    }
+                    catch (NotFoundException ex)
+                    {
+                        throw new NotFoundException($"Couldn't find merge base. Please check your network connection and try again.", ex);
+                    }
                 }
 
                 var file = await ExtractToTempFile(repo, pullRequest.Number, sha, fileName, encoding);

src/GitHub.InlineReviews/Services/PullRequestSessionService.cs

@@ -126,7 +126,14 @@ public async Task<string> GetPullRequestMergeBase(ILocalRepositoryModel reposito
             var headUrl = pullRequest.Head.RepositoryCloneUrl;
             var baseRef = pullRequest.Base.Ref;
             var headRef = pullRequest.Head.Ref;
-            mergeBase = await gitClient.GetPullRequestMergeBase(repo, baseUrl, headUrl, baseSha, headSha, baseRef, headRef);
+            try
+            {
+                mergeBase = await gitClient.GetPullRequestMergeBase(repo, baseUrl, headUrl, baseSha, headSha, baseRef, headRef);
+            }
+            catch (NotFoundException ex)
+            {
+                throw new NotFoundException("Couldn't find merge base. Please check your network connection and try again.", ex);
+            }
 
             return mergeBaseCache[key] = mergeBase;
         }

src/UnitTests/GitHub.App/Services/PullRequestServiceTests.cs

@@ -54,7 +54,7 @@ public async Task ExtractBase_MergeBaseAvailable_UseMergeBaseSha()
         }
 
         [Fact]
-        public async void MergeBaseNotAvailable_ThrowsFileNotFoundException()
+        public async void MergeBaseNotAvailable_ThrowsNotFoundException()
         {
             var baseFileContent = "baseFileContent";
             var headFileContent = "headFileContent";
@@ -64,7 +64,7 @@ public async void MergeBaseNotAvailable_ThrowsFileNotFoundException()
             var headSha = "headSha";
             var mergeBaseSha = null as string;
             var head = false;
-            var expectMessage = $"Couldn't find merge base between {baseSha} and {headSha}.";
+            var expectMessage = $"Couldn't find merge base. Please check your network connection and try again.";
             var mergeBaseException = new NotFoundException(expectMessage);
 
             var ex = await Assert.ThrowsAsync<NotFoundException>(() => ExtractFile(baseSha, baseFileContent, headSha, headFileContent, mergeBaseSha, mergeBaseFileContent,