unknown +variant silently ignored

[Jan Stary]

I just built "bitcoin +qt5", only to realize its "bitcoin +gui".
While build the noneexistent +qt5 variant, the default variant
was buit (which happens to be +daemon+wallet).

Is it intentional that a nonexistent variant is _silently_ ignored?
Wouldn't it be less surprising if MP stopped with an error?
Currently, a typo can result in something the user didnt want,
eventhough it was easily avoidable.

	Jan