Generate a clearer panic message

[roshanavand]

This aims to fix this issue

[CLAassistant]

All committers have signed the CLA.

[marckhouzam]

@roshanavand would you mind specifying the output before this change and after this change?

[roshanavand]

@marckhouzam Not at all.
Assuming we want to add a new flag named myFlag with shorthand h to a flagset named myFlagSet:
Before the message would read:
unable to redefine 'h' shorthand in "myFlagSet" flagset: it's already used for "myFlag" flag
Now it reads:
unable to define 'h' shorthand for "help" flag: it's already used for "myFlag" flag

Considering the linked issue, the custom flag is set first, after that the help flag creation is initiated and results to that panic.

[roshanavand]

Especially in case of using h shorthand, the message is more vague for the user as they might not know that help with the same shorthand is being initiated later and causing that error.
But I believe the new message is clearer in all the situations.