Settings - Index Pattern interface

Closes #1929 

Fixes issue where expanding search for matching index patterns and finding results would not let the user move on. Also fixes issue where the user could almost *always* move on, even without matching index patterns (introduced in #2145).

The changes in #2145 was missing handling for missing index pattern matches (the resulting ES query would return fields from *ALL* indexes), so there was nearly never a state where the user could not continue.

This also undoes #2133, as the extra call to `refreshFieldList` was not needed to fix #1929 

As an added UI touch, the default `fetchFieldsError` is a *Loading* message, which the watcher will reset - this stops the flash of the *Time-field name* input when the page is first loaded.

Previously (when #1929 was opened):

- Choose *Use event times to create index names*
- Find no matches
- Click *Expand Search*
- Find matches, but don't show *Time-field name* input

Currently (after #2145):

- Choose *Use event times to create index names*
- Find no matches
- Show *Time-field name* input anyway

This PR:

- Choose *Use event times to create index names*
- Find no matches
- *Don't let the user move on*
- Click *Expand Search*
- Find matches, show *Time-field name* input, as expected