bubble up some docker errors to user

currently:

* container ran out of memory (code 137)
* container exited with other code != 0
* unable to pull image (auth/404)

there may be others but this is a good start (the most common). notably, for
both hot and cold these should bubble up (if deterministic, which hub isn't
always), and these are useful for users to use in debugging why things aren't
working.

added tests to make sure that these behaviors are working.

also changed the behavior such that when the container exits we return a 502
instead of a 503, just to be able to distinguish the fact that fn is working
as expected but the container is acting funky (400 is weird here, so idk).

removed references to old IsUserVisible crap and slightly changed the
interface for RunResult for plumbing reasons (to get the error type,
specifically).

fixed an issue where if ~/.docker/config.json exists sometimes pulling images
wouldn't work deterministically (should be more inline w/ expectations now)

closes #275

api/server/server_test.go

@@ -118,7 +118,7 @@ func TestFullStack(t *testing.T) {
 		{"get myroute2", "GET", "/v1/apps/myapp/routes/myroute2", ``, http.StatusOK, 0},
 		{"get all routes", "GET", "/v1/apps/myapp/routes", ``, http.StatusOK, 0},
 		{"execute myroute", "POST", "/r/myapp/myroute", `{ "name": "Teste" }`, http.StatusOK, 1},
-		{"execute myroute2", "POST", "/r/myapp/myroute2", `{ "name": "Teste" }`, http.StatusInternalServerError, 2},
+		{"execute myroute2", "POST", "/r/myapp/myroute2", `{ "name": "Teste" }`, http.StatusBadGateway, 2},
 		{"get myroute2", "GET", "/v1/apps/myapp/routes/myroute2", ``, http.StatusOK, 2},
 		{"delete myroute", "DELETE", "/v1/apps/myapp/routes/myroute", ``, http.StatusOK, 1},
 		{"delete myroute2", "DELETE", "/v1/apps/myapp/routes/myroute2", ``, http.StatusOK, 0},