Add fallback error text

[prymitive]

If the JSON object has no error key then we end up with empty error message, add fallback.

[github-actions]

This pull request was validated by pint.

✔️ No problems found

Stats

Stat	Value
Version	v0.60.0-25-g0090686
Number of rules parsed	4
Number of rules checked	0
Number of problems found	0
Number of offline checks	0
Number of online checks	0
Checks duration	0

Problems

No problems reported

[github-actions]

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   110.4m ± 1%   110.4m ± 1%       ~ (p=0.971 n=10)
CheckRules-4     8.521 ± 2%    8.553 ± 2%       ~ (p=0.393 n=10)
geomean         970.1m        971.9m       +0.18%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   53.95Mi ± 0%   53.95Mi ± 0%       ~ (p=0.579 n=10)
CheckRules-4    937.9Mi ± 0%   934.9Mi ± 0%  -0.33% (p=0.005 n=10)
geomean         224.9Mi        224.6Mi       -0.16%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   434.9k ± 0%   434.9k ± 0%       ~ (p=0.697 n=10)
CheckRules-4    13.93M ± 0%   13.92M ± 0%  -0.10% (p=0.002 n=10)
geomean         2.462M        2.460M       -0.05%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.37%. Comparing base (66614e1) to head (0090686).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1013      +/-   ##
==========================================
+ Coverage   94.34%   94.37%   +0.02%     
==========================================
  Files          95       95              
  Lines        9575     9580       +5     
==========================================
+ Hits         9034     9041       +7     
+ Misses        382      381       -1     
+ Partials      159      158       -1     

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