Check for Vec subaccess in NamedComponent and throw a nicer error. (backport #2907) (#2928)

* Check for Vec subaccess in NamedComponent and throw a nicer error. (#2907)

This would previously end up throwing an exception later, when trying
to create a component name and realizing that it was invalid. Instead,
this detects Vec subaccesses early, and gives a more precise error and
suggestion.

(cherry picked from commit d8c30961

)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Resolve backport conflicts

Co-authored-by: Mike Urbach <mikeurbach@gmail.com>