[AdamMagued]

Fixes #63

Solution

• Updated isError to verify whether the provided type implements the standard error interface via types.Implements.
• Added unit tests covering custom structs and types implementing error.

Verified with test suite.

[tomarrell]

Thanks for this. It also flagged functions that return a custom error type, like func f() ext.CustomErr, where wrapping isn't possible. I've pushed a commit so it only flags them when the function returns error.