Hadrian: When failing to match a key-value entry give a example entry.

Today I tried

andi@horzube:~/ghc_ww_othercon$ hadrian/build -j6 -o_validate --flavour=validate "ghc.ghc.stage1.hs.opts += -ddump-simpl -ddump-to-file"
Up to date
user error (failed to apply key-value settings: - error while setting `ghc`.ghc`.stage1`.hs`.opts: matchOneOf: no match
)

Confusing the order of stage1 and ghc. Giving an example of a correct entry might make this easier to recognize immediately.

Maybe stage1.ghc-bin.ghc.hs.opts += -Werror or something.

This isn't particular high priority so I won't change this right away but putting up a ticket so I don't forget about this idea.