Should warn unknown if named format is not closed

[exoego]

Closes #434

I believe that: if {named_format is NOT closed (missing }) , the below while loops infintely and bloat the string variable sub, thus causes memory allocation error.

vue-i18n/src/format.js

Lines 48 to 51 in 94583e5

	while (char !== '}') {
	sub += char
	char = format[position++]
	}

Personally, I prefer error to be thrown, so that one can notice the invalid named format while development.
But I follow the current design: logging warn rather than throwing error.

[codecov-io]

Codecov Report

Merging #435 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #435      +/-   ##
==========================================
+ Coverage   95.91%   95.91%   +<.01%     
==========================================
  Files           9        9              
  Lines         636      637       +1     
==========================================
+ Hits          610      611       +1     
  Misses         26       26

Impacted Files	Coverage Δ
src/format.js	100% <100%> (ø)	⬆️

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[kazupon]

Good job! 👍
Thanks!