Add pipefail to aws-mfa to exit on profile errors

[ikuwow]

Summary

• Add pipefail option to bin/aws-mfa so the script exits immediately when
  aws sts get-caller-identity fails (e.g., "The config profile could not be found").

Why

The script pipes aws | jq | sed on line 51. Without pipefail, the
pipeline exit status comes from the last command (sed), which always
succeeds — masking the AWS CLI error and letting the script continue with
an invalid ARN.

Verification

• shellcheck bin/aws-mfa passes with no warnings.
• Ran aws-mfa -p nonexistent-profile-12345 and confirmed the script exits immediately with:

  Using `nonexistent-profile-12345` profile
  aws: [ERROR]: The config profile (nonexistent-profile-12345) could not be found
  Exit code: 255
  

Closes #31