Improve error handling on the Search (by pid) page

[jypeter]

When I use the Dataset Errata - PID page, things work OK if the search returns a result, e.g with: hdl:21.14100/2aeb09d8-8a92-4277-afc6-4ae258c69316

Unfortunately, if you search something that does not exist (e.g replace 16 with 17 in the previous pid):

• you get a cryptic error popup, An error occurred whilst calling the PID handle service, rather than a PID not found message
• when you click on OK in the popup, the Searching PID handle service ... please wait popup stays on the page like some kind of guru meditation, and the only way to get out of this is to reload the page

Same problem on FF and Chrome

Same problem if you type something else than a PID (I naively started trying a free form text search). It would be nicer in that case to have a popup displaying the allowed type of input

Also, what would be nice (if doable), would be to start the Search by typing Return, rather than having to click on Search. Right now, nothing happens when you type Return