Need to improve error message when basic auth flags are used with identity token flags

[wangxiaoxuan273]

After #1294 is completed, cmd.MarkFlagsMutuallyExclusive is used to prevent --identity-token / --identity-token-stdin to be used with --username/ --password / --password-stdin. However, the default error message is confusing for the users. We need to improve the error messages.

[shizhMSFT]

@wangxiaoxuan273 Can you note down the current error message in this issue?

[wangxiaoxuan273]

Current error message: Error: if any flags in the group [password identity-token identity-token-stdin password-stdin] are set none of the others can be; [identity-token password] were all set

[qweeah]

Also need to improve MarkFlagsRequiredTogether as it's used by mTLS flag parsing.

[qweeah]

The returned error should be compliant to the error guideline proposed in #1163