Don't allow negative value to be passed for --max-results flag

[mmourick]

What

• Add PreRun to list subcommand to validate user input is non-negative

Why

• Fixes a runtime error when negative values are passed.

References

• Closes Negative max-results yields an out of bound #32

[bschaatsbergen]

Would using a PreRun in Cobra not be nicer to handle the negative value, in that case we can show our own error message instead of a Go exception?

[mmourick]

@bschaatsbergen the Cobra message wasn't too bad imo, but I agree our own message is nicer and more consistent with the rest of the tool.

Cobra validation:

Error: invalid argument "-5" for "--max-results" flag: strconv.ParseUint: parsing "-5": invalid syntax

PreRun validation implemented in 4fe90e7

error: --max-results cannot be negative: see `tfversion list -h` for help and examples

[bschaatsbergen]

LGTM 🚀