Clarification and motivation

This issue is particularly important for the FA2 standard, as this allows for errors to be both string or pair string x, where x can be any other valid type.

Since we want to support tests for an FA2 contract regardless of which of the two types it's using in its error, we need a simple way declare this in tests, e.g.:

transfer entrypoint call fails and the value it fails with is either "FA2_TOKEN_UNDEFINED" string or a pair where the first item is this string (and the second one doesn't matter)

Note: this issue will mostly likely need #451 (closed).

It also closely relates to #383 (closed) and should not be started before this is complete (it may even be solved by it already.

Acceptance criteria

It's easily possibly to write a tests that expects a failure for a specific string that accepts either of this as valid:

1. the string itself on its own
2. that string in a pair string x where x can be anything