fix: createError fallback to statusMessage

[antfu]

Currently, when using Nuxt, the createError does not generate meaningful error messages if only statusMessage is provided (which I see is more commonly been used in the Nuxt code base)

I would think it could be better to serve statusMessage if message is not provided. Ideally, maybe we could have better format as `${input.statusCode}: ${input.statusMessage}` but I will leave it for discussions

[codecov-commenter]

Codecov Report

Merging #25 (82026e2) into main (8d39032) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   93.95%   93.95%           
=======================================
  Files          10       10           
  Lines         182      182           
  Branches       49       42    -7     
=======================================
  Hits          171      171           
  Misses         11       11           

Impacted Files	Coverage Δ
src/error.ts	89.65% <100.00%> (ø)

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[pi0]

Thanks!