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Handle non ascii characters in keys with an error.

Instead of a hard to handle UnicodeEncodeError exception, check the keys and
print a human readable error. Turn more format strings into utf-8 to avoid the
exception.

Fixes #116

For keys:
* "CONTROLS_FOREACH_HELPURL": "https://github.com/google/blockly/wiki/Loops#for-each",
* "MATH_CONSTRAIN_HELPURL": "https://en.wikipedia.org/wiki/Clamping_%28graphics%29",
* "LISTS_ISEMPTY_HELPURL": "https://github.com/google/blockly/wiki/Lists#is-empty",

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