Graceful handling of wrong user input

[flavio]

This is kinda related with #1, right now the code converts the limit value provided by the user to a Quantity object.
The conversion is done by using the MustParse function, which panics when the user input is wrong.

The code should instead use the ParseQuantity function which returns an error. The code should handle errors and tell the user which input provided was not valid. This allows easier remediation of wrong input by the user.