fix: improve error messages when validating extensions

This commit changes the messages produced when validating extension names and
values, including the offending name or value in the message, and including
text from the CloudEvent specification, or a link to the type system.

Fixes: #364

Signed-off-by: Lance Ball <lball@redhat.com>

Author: lance

Diff for: src/event/cloudevent.ts

@@ -106,13 +106,18 @@ export class CloudEvent implements CloudEventV1, CloudEventV03 {
       // Extension names should only allow lowercase a-z and 0-9 in the name
       // names should not exceed 20 characters in length
       if (!key.match(/^[a-z0-9]{1,20}$/) && strict) {
-        throw new ValidationError("invalid extension name");
+        throw new ValidationError(`invalid extension name: ${key}
+CloudEvents attribute names MUST consist of lower-case letters ('a' to 'z')
+or digits ('0' to '9') from the ASCII character set. Attribute names SHOULD
+be descriptive and terse and SHOULD NOT exceed 20 characters in length.`);
       }
 
       // Value should be spec compliant
       // https://github.com/cloudevents/spec/blob/master/spec.md#type-system
       if (!isValidType(value) && strict) {
-        throw new ValidationError("invalid extension value");
+        throw new ValidationError(`invalid extension value: ${value}
+Extension values must conform to the CloudEvent type system.
+See: https://github.com/cloudevents/spec/blob/v1.0/spec.md#type-system`);
       }
 
       this[key] = value;