Make base64 encoding target independent in KeyIdMemStore in wasm bindings

[wulfraem]

Description of change

Makes base64 encoding in KeyIdMemStore, methodDigestToString work for node and web targets. By using new dependency base64-arraybuffer.

Also adds smaller improvements for storage related errors.

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

Tested in browser and against local wasm tests.

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