Fix three pretty exception messages

Author: mpilgrem

src/Stack/Setup.hs

@@ -358,7 +358,7 @@ data SetupPrettyException
 
 instance Pretty SetupPrettyException where
     pretty (GHCInstallFailed ex step cmd args wd tempDir destDir) =
-         flow "Error: [S-7441]"
+         "[S-7441]"
       <> line
       <> string (displayException ex)
       <> line

src/Stack/Upload.hs

@@ -56,13 +56,13 @@ data UploadPrettyException
 
 instance Pretty UploadPrettyException where
     pretty AuthenticationFailure =
-           "Error:" <+> "[S-2256]"
+           "[S-2256]"
         <> line
         <> flow "authentification failure"
         <> line
         <> flow "Authentication failure uploading to server"
     pretty (ArchiveUploadFailure code res tarName) =
-           "Error:" <+> "[S-6108]"
+           "[S-6108]"
         <> line
         <> flow "unhandled status code:" <+> fromString (show code)
         <> line