Feature: show name of variable if conversion fail

[LuckyTurtleDev]

The error type does not store information about the name of a variable, if a conversion fail (expect of bool).

So the user see only a error message that a conversion has failed, but not witch environment variable was converted.
This make it more difficult for the user to find the wrong environment variable.

error loading Environment Variable:
 Error(InvalidInteger(ParseIntError { kind: InvalidDigit }))