redirect error output to stderr

[mbacchi]

Stderr should be separated from stdout on error.

I'm opening a PR to make it do this properly:

`[mbacchi@hostname git-secrets]$ ./git-secrets --scan -r . 2>err 1>out [mbacchi@hostname git-secrets]$ cat out [mbacchi@hostname git-secrets]$ cat err ./creds:1:AKIAISITMFSJISKWFAJQ

[ERROR] Matched one or more prohibited patterns

Possible mitigations:

• Mark false positives as allowed using: git config --add secrets.allowed ...
• Mark false positives as allowed by adding regular expressions to .gitallowed at repository's root directory
• List your configured patterns: git config --get-all secrets.patterns
• List your configured allowed patterns: git config --get-all secrets.allowed
• List your configured allowed patterns in .gitallowed at repository's root directory
• Use --no-verify if this is a one-time false positive`