net_plugin::plugin_startup gives more information when it attempts to…

[banshee]

If net_plugin::plugin_startup fails to bind, the error message in the log is mysterious; all you know is that something, somewhere failed to bind to a port.

After the patch, you get:

2018-08-12T16:09:51.860 thread-0   net_plugin.cpp:3015           plugin_startup       ] net_plugin::plugin_startup failed to bind to port 9876
2018-08-12T16:09:51.878 thread-0   main.cpp:145                  main                 ] std::exception

Before the patch, you get no indication of what code might be causing the problem:

2018-08-12T16:19:45.887 thread-0   http_plugin.cpp:401           plugin_startup       ] start listening for http requests
2018-08-12T16:19:45.909 thread-0   main.cpp:131                  main                 ] Throw location unknown (consider using BOOST_THROW_EXCEPTION)
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::system::system_error> >
std::exception::what: bind: Address already in use

Helps people diagnose problems with issues like #3999.