Hi pvpalval,
Out of box, you cannot create alerts based on "Error Details:" in Wily.
But since you mentioned that you have created metrics using custom PBD's, I would suggest you to bring specific types of errors [only generic classification is possible] by bringing them to specific custom node in the investigator, and then creating alerts on top of this.
To elaborate, you can direct in the PBD file for all HTTP error code error into some specific path such as - "Host1|WebSphere|Agent1|Errors|HTTPError:Errors Per Interval".
And now you can set an alert where danger threshold for this you can put as 1, which will trigger the alert as soon as any HTTP error code is observed. Similarly you can categorize your SQL or null pointer exceptions into one.
To get an alert on specific type of error, you can do using EPAgent -Logreader and enable ERROR logging into your application logs. LogReader plugin will read strings at end of file, and would change counter from say 0 to 1, which you can use to trigger alerts,
Hope it helps.
Regards,
Vaibhav