Index pattern creation UX: Surface "no matching indices" error more clearly

[cjcenizal]

Taken from #9991 (comment):

The message "Unable to fetch mapping. Do you have indices matching the pattern?" is too disconnected from the source of the error (the bad index pattern field). I think the solution is:

• This button should just become disabled without changing text.
• The index pattern field should be outlined in red.
• There should be an error message below it that states, "Do you have indices matching this pattern? We couldn't fetch the mapping." (And what does "couldn't fetch the mapping mean"?" Doesn't it make more sense to say, "couldn't find any matching indices?")

[cjcenizal]

Addressed by #13454