refactor: Error from primitives to contract library

[Daanvdplas]

The Error type is only used in runtime via tests, and the fact that a primitives crate is referring to pop api on line 16 makes it seem off.

I think the tests that use this error should probably live in pop-api/integration-tests, especially as it already takes a dev-dependency on the runtime. Then this error should only need to be defined in the pop-api crate, where it probably belongs as it is a contract-side only representation of the errors which exist in the runtime.

If a type isnt being used by both a contract AND the runtime, it probably doesnt need to be in this crate.

[evilrobot-01]

Should also be checked in terms of runtime size, as you might effectively be compiling stuff into the runtime which is unnecessary, just for the convenience.

[Daanvdplas]

Closing in favour of #91