Improve error when reserved word used as label

Before this commit error looks like (for input `start = break:'a'`)

> Expected "!", "$", "&", "(", "*", "+", ".", "/", "/*", "//", ";", "?", character class, code block, comment, end of line, identifier, literal, or whitespace but ":" found.

After this error looks like

> Label can't be a reserved word "break".