In trying to give error diagnostics with template definitions of invalid
types, the user encounters some pretty confusing error messages at
first. There is no way to get the compiler to give exactly the
diagnostics we want in a nice readable error message, so we are putting
some verbose instructions as comments in the code. However, a user might
not know to look at the source code since the error happens deep in an
unfamiliar (and complicated) class. Thus, add (yet another) error at the
front that gives a (hopefully) clear indication to look at the source
code for help in understanding the errors.