No completion if the cache has action name that are partially matched

[curtisman]

Repro:

• Populate the cache with open bbc webpage, where the explainer will indentify "open" and "webpage" as subphrase that translate to "browser.openWebPage".
• Then completion will fail with open because it couldn't transform the action name with only the first part of the subphrase.

#1478 is a partial fix that stop the exception to kill all completion. But ideally,

1. we are able to partiall match "open" and determine that we will get "browser.openWebPage", based on the only possible transform with prefix open.
2. provide web site complete, either from "memory" or agent returned known web sites.