[FR] Communicate when a zoekt search limit is hit #504

Problem

We have a set of configurable limits that Zoekt uses to determine when to terminate a search:

SHARD_MAX_MATCH_COUNT // Maximum number of matches: skip all processing an index shard after we found this many non-overlapping matches.
TOTAL_MAX_MATCH_COUNT //  Maximum number of matches: stop looking for more matches once we have this many matches across shards.
ZOEKT_MAX_WALL_TIME_MS // Abort the search after this much time has passed.

Currently, there is no UX or logs to signal when these limits are hit, resulting in inaccurate results and user confusion. This is especially bad for queries that need to be exhaustive, e.g.,

• "does X exist anywhere?" (this could hit the max wall time)
• "how many instances of Y exist?" (this could hit both the max wall time and the max match counts)

Proposed Solution

Add some UX to communicate when a search limit has been hit and why.

Related #495