Configuration files are parsed into sections of key/value pairs when
they are read, but at that point values are still arbitrary bytes.
Only when a value is accessed by various parts of the code do we know
its expected type and syntax, so values are parsed at that point.
Let’s make a new error type for this latter kind of parsing error,
and add a variant to the common HgError so that most code can propagate it
without much boilerplate.