Fix the bug with the enums having "Error" variant.

It was impossible to have an enum with a variant called "Error", for example:

enum A { Error = 1, }

Would throw an error due to the ambiguity of using Self::Error in the TryFrom implementations and a possible reference to A::Error (A is Self).

This effectively removes the ambiguity by specifically referring to the TryFrom implementation in the current scope, instead of the enum.

This commit is an advancement of !7 (diffs) in some way.