Remove exceptions thrown during issuer validation. Add validation source.

[iNinja]

Issuer validation: Remove exceptions and add validation source

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Removed ArgumentNullException thrown as part of issuer validation, and added source.

Description

• Replaced instances that threw ArgumentNullException with a return of IssuerValidationResult containing the exception information.
• Addressed TODO around identifying the source of the issuer validation (configuration, validation parameters: valid issuer or among list of valid issuers).
• Added tests around all the previous cases.

[brentschmaltz]

We will have to generalize ValidationSource.
We should also consider other validators before we make a choice.