The big change here is that `QiskitError` is added to the API
documentation, which causes 400--500 previously failing Sphinx lookups
(stemming from "Raises:" documentation) to now succeed.  This commit
also corrects several other places where exceptions were not being fully
documented (and in several cases makes the imports more convenient as
well), and corrects a couple of places with incorrect references to
exceptions.

(cherry picked from commit 982807e)

# Conflicts:
#	docs/apidocs/exceptions.rst
#	qiskit/dagcircuit/__init__.py
#	qiskit/passmanager/__init__.py
#	qiskit/transpiler/__init__.py

This commit fixes several merge conflicts in the backport. They were
mostly minor differences between main and the stable branch that needed
to be corrected.