ErrorPageFilter contained an optimisation for looking up the path
of an error page by exception type. For cases where there was no
mapping for the type of the exception that was thrown but there
was a mapping for one of its super classes, it was intended to
speed up the lookup. Unfortunately, there was a bug in the
implementation which meant that the optimisation had no effect.

Analysis with JMH reveals that for an Exception with a deep type
hierarchy, such as Spring Framework's UnsatisfiedDependencyException,
and an error page mapping for Exception, searching up the hierarchy
until a mapping is found takes 0.0000001s. With the same mapping,
a lookup for Exception takes 0.00000001s, i.e. it's 10x faster.
The optimisation, when correctly implemented, brings the time for
UnsatisfiedDependencyException down to 0.00000001s and into line
with a lookup for Exception. However, the amount of time involved is
so small compared to the overall time spent processing a request that
the added complexity of the optimisation is not justified.

Closes gh-7010