feat(core): error if tcp perm denied

While in live-preview mode connecting to an already used port or a port
which is reserved for elevated users, fleck now logs the resulting error
from 'http.ListenAndServe' via the 'logger.LError' utility.

Fleck's output while trying to serve at port 80 without elevated
privileges:

    $ fleck --port=80 --live-preview README.md
    2023/04/24 09:57:44 info: starting live preview
    2023/04/24 09:57:44 info: listening on http://localhost:80/README.html
    2023/04/24 09:57:44 error: listen tcp :80: bind: permission denied
    exit status 1

core/core.go

@@ -116,7 +116,7 @@ func LivePreview(fileName string) {
 
 	logger.LInfo("listening on http://localhost:" + port + "/" + file + ".html")
 	spawnDefaultBrowser("http://localhost:" + port + "/" + file + ".html")
-	http.ListenAndServe(":"+port, nil)
+	logger.LError(http.ListenAndServe(":"+port, nil).Error())
 }
 
 // watches for changes in a file, recompiles the file if a change occurs, can be exited via <C-c>