Give stopword feedback in search

[gnprice]

Zulip search ignores some very common words ("stopwords"), like "the" or "a". In particular, if you try a search query where all your query words are stopwords, there will be no results.

The web app gives some explicit feedback when this happens:

stopword only:

No search results.
Common words were excluded from your search:
the

stopword and another word, but no results:

No search results.
Common words were excluded from your search:
the klsfjakldfj

no results, but no stopword:

No search results.

We should do something similar. In email, a user writes:

On the desktop, if you search for "the", it tells you this happened. On the app, it doesn't. I was very confused why "test" returned seemingly several thousand messages whereas "that" returned zero.