Improve UX when API port is already in use

[lidel]

Current State

If I already have something running on port :5001 ipfs-desktop fails to start and generic error is displayed (3b7c781):

Actual error can be found in logs:

Error: Command failed: /home/lidel/project/ipfs-desktop/node_modules/go-ipfs-dep/go-ipfs/ipfs daemon --migrate=true --routing=dhtclient
Error: serveHTTPApi: manet.Listen(/ip4/127.0.0.1/tcp/5001) failed: listen tcp4 127.0.0.1:5001: bind: address already in use

Initializing daemon...
go-ipfs version: 0.4.18-
Repo version: 7
System version: amd64/linux
Golang version: go1.11.1
Swarm listening on /ip6/::1/tcp/4001
Swarm listening on /p2p-circuit
Swarm announcing /ip6/::1/tcp/4001
Received interrupt signal, shutting down...
(Hit ctrl-c again to force-shutdown the daemon.)

    at makeError (/home/lidel/project/ipfs-desktop/node_modules/execa/index.js:174:9)
    at Promise.all.then.arr (/home/lidel/project/ipfs-desktop/node_modules/execa/index.js:278:16)
    at process._tickCallback (internal/process/next_tick.js:68:7)

Proposed Improvements

IPFS Desktop should detect Error: serveHTTPApi: [...] bind: address already in use and

• at minimum: display failed: listen tcp4 127.0.0.1:5001: bind: address already in use
• nice to have: display UI for changing API port and retrying
• extra: find a free port and display it in the UI so user does not guess what is safe to use

[lidel]

Continued in #912