html/template,text/template: use errors.New instead of fmt.Errorf

[apocelipes]

For non-formatted strings "fmt.Errorf" and "errors.New" have the same result,
but the former is slower and allocates more memory.

So it is better to call "errors.New" directly for non-formatted strings.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from kyose chang:

Patch Set 1:

(1 comment)

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