🐛fix: preview not show bug due to incorrect hosts file

[CallanBi]

I am a Gridea user and I found a bug caused by a boundary case in my usage.

When 127.0.0.1 is not mapped to localhost in the local hosts file (common when proxy applications modify the file, e.g. Proxifier), the rendering service does not start.

The local hosts file that causes this problem looks like this.

# Proxifier
127.0.0.1 www.proxifier.com
127.0.0.1 proxifier.com

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##

This pull request is a fix for this boundary case. There are two changes from the previous version.

1. no incremental port switching will be attempted after localhost is not found.
2. give a friendly prompt after a failed preview renders, telling the user to check the hosts file.

[EryouHao]

Great MR! Thanks @Moltemort