Make string->ID conversion fail if ID is invalid

[MattWindsor91]

The current IDFromString converter doesn't reject invalid IDs. Ideally, as the ways in which an ID can be invalid are growing, it should be replaced with two converters:

• a validating converter that basically just behaves as NewID(strings.Split(..., ".")) and returns an error, for use when unmarshalling and otherwise accepting IDs from the outside world;
• a 'trusting' converter that just returns the empty ID (or suchlike) on error, to be used in tests and places where we 'know' the ID is safe.

[MattWindsor91]

id.TryFromString implements this now.