Adapt StorageError to be more generic

[olivereanderson]

Description of change

This PR renames and moves StorageError to identity_core so that it may be used in more contexts where the single struct error design pattern is desirable. A first example of this is the additional use in SignatureVerificationError in identity_jose.

Also uses Debug + Display as trait bounds directly on the error kind and removes the ErrorKind trait in order to reuse existing traits and not introduce too many new traits.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Runs under cargo test locally and the bindings build without problems.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for 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
• 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