scan_log_for_errors: check that regex is correct (#9815)

## Problem

I've noticed that we have 2 flaky tests which failed with error:
```
re.error: missing ), unterminated subpattern at position 21
```

- `test_timeline_archival_chaos` — has been already fixed 
- `test_sharded_tad_interleaved_after_partial_success` — I didn't manage
to find the incorrect regex

[Internal link](https://neonprod.grafana.net/goto/yfmVHV7NR?orgId=1) 

## Summary of changes
- Wrap `re.match` in `try..except` block and print incorrect regex

test_runner/fixtures/pageserver/allowed_errors.py

@@ -25,8 +25,14 @@ def scan_pageserver_log_for_errors(
 
             # It's an ERROR or WARN. Is it in the allow-list?
             for a in allowed_errors:
-                if re.match(a, line):
+                try:
-                    break
+                    if re.match(a, line):
+                        break
+                # We can switch `re.error` with `re.PatternError` after 3.13
+                # https://docs.python.org/3/library/re.html#re.PatternError
+                except re.error:
+                    print(f"Invalid regex: '{a}'", file=sys.stderr)
+                    raise
             else:
                 errors.append((lineno, line))
     return errors

test_runner/fixtures/utils.py

@@ -495,8 +495,14 @@ def scan_log_for_errors(input: Iterable[str], allowed_errors: list[str]) -> list
 
             # It's an ERROR or WARN. Is it in the allow-list?
             for a in allowed_errors:
-                if re.match(a, line):
+                try:
-                    break
+                    if re.match(a, line):
+                        break
+                # We can switch `re.error` with `re.PatternError` after 3.13
+                # https://docs.python.org/3/library/re.html#re.PatternError
+                except re.error:
+                    log.error(f"Invalid regex: '{a}'")
+                    raise
             else:
                 errors.append((lineno, line))
     return errors