enhance vague error when creating ext-jwt-signer

[dovholuknf]

Tonight I tried to effectively duplicate an ext-jwt-signer when testing. The name and audience (red) were different but the issuer, claims prop, client id, ext auth url were all the same (green).

On save of "auth1" a vague error pops out

Looking at dev tools the error is much more clear and obvious that the actual error was the issuer field was duplicated.

{
    "error": {
        "cause": {
            "field": "issuer",
            "reason": "duplicate value 'https://dev-k0qmk4wpyslypklr.us.auth0.com/' in unique index on externalJwtSigners store",
            "value": "https://dev-k0qmk4wpyslypklr.us.auth0.com/"
        },
        "code": "COULD_NOT_VALIDATE",
        "message": "The supplied request contains an invalid document or no valid accept content were available, see cause",
        "requestId": "rvOqok3MK"
    },
...