Contract execution return explicit errors instead of contract_failure

[bchamagne]

Description

Contract execution error used to return always the same error. No more.
This is a first step in direction of #1197

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests + Playground

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[samuelmanzanera]

I think sooner or later, we have to implement #965 to uniform and reduce strings in validation