rust: Add a `ConfigValueParseError` variant to common errors 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. Differential Revision: https://phab.mercurial-scm.org/D10009