Live Preview fixes

[redmunds]

This is for #6122. Also is for #5687. And at least parts of #6830.

I also found a repeatable recipe that was causing connection errors when changing files, so I fixed the case of relaunching browser after "Relaunch Chrome" dialog is shown. See inline comments for more info.

[redmunds]

The 404 error message provides a url that is not added to error message. This case I saw ended up not being related to this code, but this is good info to display.

[jasonsanjose]

Looks good.

[redmunds]

@jasonsanjose Do you want to take a look? I think you were the last one to update this code.

[redmunds]

Seems like a pending _openDeferred should be rejected before disconnecting. This makes the scenario I was using seem to be more reliable when switching files.

[redmunds]

@jasonsanjose Re-assigned to @ingorichter

[redmunds]

@ingorichter @jasonsanjose Can someone make a first pass through this pull request? We should try to get this in for Release 37, and I have a few questions that still need to be resolved. Thanks.

[jasonsanjose]

Remove each addition of _loadAgentsPromise = null; and instead setup the following:

result.always(function () {
  _loadAgentsPromise = null;
}

[redmunds]

Much better. Thanks.

[jasonsanjose]

@redmunds initial review complete

[redmunds]

In theory, the _onDocumentSaved() event handler could be called again before agents and page are done, but this is probably safe for now since I can't figure out how to hit this code. HTML adn CSS pages have liveEditingEnabled set to true, so the bail out. I tried attaching a JS file, but agents.network.wasURLRequested() returns falsey.

[redmunds]

@jasonsanjose I added better handling of unloadAgents().

[redmunds]

New call to open() causes a circular dependency.

[redmunds]

@jasonsanjose Ready for another review.

[ingorichter]

Switching the documents will start a new session every time. This seems to be alright now.
F.Y.I. I don't think that this a real use-case, but starting and stopping Live Preview but constantly clicking the icon will result in an error after 4-6 times of doing so. I get an error and the console log shows Inspected frame has gone Object {code: -32000, message: "Inspected frame has gone"}

[njx]

@ingorichter That last issue is worth filing - the fact that we get easily confused if you disconnect/reconnect rapidly. Some of the students were running into this at MissionBit.

[jasonsanjose]

I'm questioning now whether or not this is useful. At this point, we've spun up the server and created the main live document, but we just haven't established a connection to Chrome. Either it's not running or we can't connect to the debug port. Does this call to close even make sense?

If not, then I think we can remove it and your change below goes back to _openInterstitialPage without the timeout as you said.

[jasonsanjose]

Unfortunately, I can still hit the "Unable to load Live Development page" error message following this comment #6889 (comment).

Is it worth it to try changing reconnect to reuse the connection instead of a full teardown and restart?

[jasonsanjose]

@redmunds I went ahead and changed how we load a new HTML page if live preview is already open. See #7119. I still need to scrub the tagged live preview issues to see which ones this fix addresses.

[jasonsanjose]

Looks great. Merging.