Add debugging help to VisualEditor

VE doesn't use the debug log at all, it seems.  This adds a single
wfDebugLog call so that the person using the debug log can see that it
actually does hit VE.

Also, returns better information for parsoid errors.

For example, without it, a parsoid misconfiguration of setInterwiki()
gives:

    Error loading data from server: parsoidserver-http-bad-status:
    500. Would you like to retry?

with it, you get something like the following:

    Error loading data from server: parsoidserver-http-bad-status:
    500: Page Fetch failure for "BADURL": Error: getaddrinfo
    ENOTFOUND. Would you like to retry?

or

    Error loading data from server: parsoidserver-http-bad-status:
    500: Page Fetch failure for "BADURL": 404. Would you like to
    retry?

Bug: 39057
Bug: 43147
Bug: 63149
Change-Id: I52db9fac42d7c0228b93ce3caec470fff98254f0
Merge "Add debugging help to VisualEditor"