[JUJU-759] Change error types to use new ConstError

[barrettj12]

Following #52, this library now supports errors.Is, errors.As, and errors.Unwrap from Go's standard errors library. This PR changes the IsX functions from errortypes.go to use errors.Is under the hood:

• We've defined a new error type ConstError, whose values are "sentinel errors" representing a certain error type.
• We've defined errWithType, which is an Err bundled with its error type errType ConstError. errWithType implements an Is method that compares its argument to errType. This ensures e.g. Is(e, NotFound) works as expected.
• All the individual structs for different error types, e.g. timeout, notFound have been replaced with errWithType.
• For X ConstError, Xf and NewX now return e errWithType where e.errType is X.
• IsX functions are now implemented by comparison to X.

This is structurally quite a big change, but the errors should behave exactly as before (all the tests in errortypes_test.go pass).

[jujubot]

Can one of the admins verify this patch?

[jujubot]

Can one of the admins verify this patch?

[barrettj12]

!!build!!

[tlm]

lgtm

[hpidcock]

Just one nagging question.

[barrettj12]

$$merge$$