Better NoValidCandidatesError messages.

[mbleigh]

Before

Error: FAILED_PRECONDITION: Generation resulted in not a single candidate matching provided output schema.

After

Error: FAILED_PRECONDITION: Generation resulted in not a single candidate matching provided output schema.

Candidate[0] Error: INVALID_ARGUMENT: Schema validation failed. Parse Errors:

- (root): must have required property 'name'

Provided data:

{
  "displayName": "Some Name"
}

Required JSON schema:

{
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    }
  },
  "required": [
    "name"
  ],
  "additionalProperties": true,
  "$schema": "http://json-schema.org/draft-07/schema#"
}