Motivation

* `check_benchmark_thresholds.sh` was updated to check for exit code 2
from `swift package benchmark thresholds check` to distinguish
threshold regression from build errors.
* It seems SwiftPM's `CommandPlugin` infrastructure always exits with code 1 when
`performCommand` throws, regardless of the error's raw value. So the
exit code is never 2, causing all regressions to fall through to the
`else` branch and be misreported as build errors, with no diff output.

Modifications

* Remove the `rc == 2` check and instead attempt `thresholds update`
for any non-zero `rc` from `thresholds check`.
* Use the result of `thresholds update` to distinguish regression
(success) from build error (failure).
* Remove `--exit-code` from `git diff` to prevent `set -uo pipefail`
from aborting the script before output is fully flushed.

Result

* Benchmark threshold regressions correctly output the `=== BEGIN DIFF
===` section again.
* Actual build errors are still correctly detected and reported.