throw error on illegal context

[Rich-Harris]

Fixes #934

[codecov-io]

Codecov Report

Merging #947 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   91.73%   91.74%   +0.01%     
==========================================
  Files         109      109              
  Lines        4005     4010       +5     
  Branches     1285     1287       +2     
==========================================
+ Hits         3674     3679       +5     
  Misses        148      148              
  Partials      183      183

Impacted Files	Coverage Δ
src/utils/reservedNames.ts	100% <ø> (ø)	⬆️
src/parse/state/mustache.ts	87.9% <100%> (+0.61%)	⬆️

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