Make magit-run* look in the magit-process-buffer for a useful error

string when reporting an error, and fall back on the less helpful "Git
failed" message only if no error message could be found in the buffer.

magit.el

@@ -1640,7 +1640,13 @@ FUNC should leave point at the end of the modified region"
 	       (magit-need-refresh magit-process-client-buffer))))
       (or successp
 	  noerror
-	  (error "Git failed"))
+	  (error 
+	   (or (save-excursion
+		 (set-buffer (get-buffer magit-process-buffer-name))
+		 (when (re-search-backward 
+			(concat "^error: \\(.*\\)" paragraph-separate) nil t)
+		   (match-string 1)))
+	       "Git failed")))
       successp)))
 
 (autoload 'dired-uncache "dired")