Add meaningful error message if multiple servers with same port
Add meaningful error message if multiple servers with same port

Lyleo/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer