The method returns a message saying that there is no valid value if ValidHigh <
ValidLow. This was useful when these fields were numbers, but since they are now
strings this no longer works.
This also replaces the slightly awkward "arguments here" reason with
"argument count" as the "what" for a typical errs.ArityMismatch
exception. It also reformats most of the constructors so that the "what"
is on the same line. This makes `grep errs.ArityMismatch **.go` more
useful as a result.
When attempting to update a read-only var return an error struct rather
than a simple string (i.e., a Go `error` type). This makes it possible
to include the var name in the error message. This builds on commit
a33ecb2d that highlights the offending var name in the stack trace but
does not include the var name in the error message. With this change the
error message includes the offending var name.
Related #255
