fix(extensions/#2484): Searching for extensions w/o network crashes Onivim

[bryphe]

Issue: When searching for extensions without a network connection, the editor would crash with:

(Invalid_argument Lwt.wakeup_exn)
Raised at file "stdlib.ml", line 30, characters 20-45
Called from file "src/process.ml", line 107, characters 12-54

Defect: There is a race condition here - we are waiting for EOF on stdout to return the promise - and claim success. However, in the error case, we'd get EOF on stdout, run Lwt.wakeup to complete the promise, and then get a call that the process has exited - and we'd call Lwt.wakeup_exn on the same promise. This triggers an invalid argument exception - we're trying to complete and then fail the same promise.

Fix: Wait until both the process has exited, and stdout has given the EOF - then we'll complete the promise. At that point, we know if the process was successful or failed, and we could do the appropriate action on the promise.

In addition, while debugging this, I saw that parsing was failing frequently when querying open-vsx - our parsing code was making the assumption that an icon field was always available, when that's not the case.

Finally, I also added an error message when the search query fails.

Fixes #2484 , and should fix #2545 as well

[bryphe]

Fixed branch name in #2552