"nikola serve": Better error message when the port is already in use.

[arunpersaud]

Requested Feature: (short description)

A better error message when the port is already in use, for example, when you start a second nikola serve instance.

Related Area: (eg. tasks, compilers, configuration, templates…)

command line interface

Do you want to contribute this yourself as a pull request? (don’t worry about it if you don’t want to/can’t — someone else can take care of it)

• [x] Yes, I have already written code for it (link if available and feasible)
• [ ] Yes, I don’t have code ready yet (that’s okay!)
• [ ] No (that’s okay too!)

Does this feature affect backwards compatibility? If yes, in what way?

No

Rationale and full description: (why should it be added to Nikola?)

Tell the user what the problem is and how to solve it (-p option). Alternatively nikola could also just try to find an open port (e.g. try 8001, 8002, etc)

[felixfontein]

Alternatively nikola could also just try to find an open port (e.g. try 8001, 8002, etc)

I would avoid automatically picking something (except when the user explicitly tells Nikola to do that). Opening unexpected ports is often not a good idea.

(But improving the error message is definitely a good idea!)