`Messages` can't be easily printed for debugging purposes

When debugging #19693 (closed) I saw that the error output of deSugarExpr was being ignored, I wanted to print it out to see if there were some errors that we were ignoring. It turns out there isn't a very convenient way to display Messages, I ended up with something like this:

 761                 unless (isEmptyMessages msgs) $ do                              
 762                   let ms = getWarningMessages msgs                              
 763                       ers = getErrorMessages msgs                               
 764                   pprTraceM "msgs" (vcat [ppr (text . show <$> ms), ppr (text . show <$> ers)])

In addition to this, there's a Show instance for MsgEnvelope but not an Outputable instance, and for Bag, there's only an Outputable instance, but no Show instance, this leads to the awkward text . show <$> ....

Proposal: Introduce an Outputable instance for Messages or at least a function debugPprMessages :: Messages -> SDoc.