print early errors that don't go through the streaming reporter

[Dspil]

Errors that do not get produced by the backend (typechecking and parsing errors) didn't pass through the streaming reporter so they were not reported. Now the verificationFailure instead of a vector of errors takes an Either as a parameter to distinguish between errors produced by the backend or not so they are printed if the streaming reporter hasn't printed them already. A simple example is the following:

package test

import "context"

func f() 

If context is not in the directories passed as arguments to Gobra, Gobra produces an error saying that this qualifier could not be derived. This kind of error did not pass through the reporter so it was not printed.

[Felalolf]

From my side, looks ok, but let joao also have a look