Add context to error messages

[piotrrzysko]

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For debugging purposes, it would be helpful to include information in error messages about what caused a given error, and preferably, where it occurred. For example, instead of returning a message like:

Invalid number. Exponent indicator has to be followed by a digit.

It would be more informative to specify the position in the input where the invalid number is, and/or attach the number to the message.