Improve promql/regexp messages

[github-actions[bot]]

Benchmark diff:

goos: linux
goarch: amd64
pkg: github.com/cloudflare/pint/cmd/pint
cpu: AMD EPYC 7763 64-Core Processor                
              │   old.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
FindEntries-4   39.27m ± 1%   39.25m ± 6%       ~ (p=0.481 n=10)
CheckRules-4     8.520 ± 2%    8.498 ± 2%       ~ (p=0.315 n=10)
geomean         578.4m        577.5m       -0.15%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.529 n=10)
CheckRules-4    978.0Mi ± 0%   978.9Mi ± 0%  +0.09% (p=0.009 n=10)
geomean         163.7Mi        163.8Mi       +0.04%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.302 n=10)
CheckRules-4    14.18M ± 0%   14.19M ± 0%  +0.06% (p=0.007 n=10)
geomean         1.367M        1.368M       +0.03%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.77%. Comparing base (b600a88) to head (741d22c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1073 +/- ## ========================================== + Coverage 94.76% 94.77% +0.01% ========================================== Files 99 99 Lines 9963 9988 +25 ========================================== + Hits 9441 9466 +25 Misses 366 366 Partials 156 156 ```

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