validate platform better by making sure we have a webserver installed here
handle missing context information better

we produce a human-readable error message that is easier to track now
instead of a "Trying to get property of non-object" error.

This can happen when there's an inconsistency in the frontend
database, which can unfortunately happen (for example when you created
a platform without a webserver because of missing error-checking or
erroneous manual data entry).

This is to help diagnosing #1896184
allow users to edit the platform path if it wasn't verified

we don't allow users to change the platform path if a makefile was
specified, however, because the verify may have ran and fail because
of a faulty makefile in which case some cruft from the drush make run
may still be lying around in the backend. we don't have this problem
if there's no makefile because it's the admin's responsability to
cleanup after themselves then.

Closes: #1898260