Clarify the error message on "{"

[vicb]

{ are used to start an ICU message.

If users have a { in their template w/o it being the start of an ICU message, the error message should be explicit (i.e. contain the escape sequence{{ '{' }}).

[vicb]

Please fill out the GH template when creating a new issue ;)

[foxjazz]

can be used to tell compiler to skip parsing (per Rob)

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