sketch: fix diff view editing of gitignore'd files and forward more http errors to logs

Fixes https://github.com/boldsoftware/sketch/issues/213

We had "sketch" git ignored, so "git add sketch/cmd/sketch/main.go" was
failing when a user was editing it in diff view. The gitignore was
incorrectly specified. ("git ls-files -i -c --exclude-standard"
returning main.go should have tipped us off, but who knew!)

Anyway, fixed that, and improved the logging.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s3ed65211dd497f76k
diff --git a/.gitignore b/.gitignore
index 2b55c68..540f606 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,4 +7,4 @@
 dist
 
 # makefile artifact
-sketch
+/sketch