[Agda] parsing reports

[Serge D. Mechveliani]

A minor note on	 Agda-2.3.2 MAlonzo:

when a type constructor is not in scope, the type checker reports
"could not parse this expression ...",
-- without explaining the reason any more precisely.
Why the checker does not report, say,  "... not in scope" ?
For example,

  open import Data.Maybe using (Maybe)   -- `just' is forgotten

  f : Maybe Bool -> Bool
  f (just _) = true
  f _        = false

causes       "could not parse (just _)"
rather than  "not in scope:  just"

For the examples with a complex expression this leads to difficulty in
finding the source of this kind of error.

Regards,

------
Sergei