make sure error.log exists and is writable on installation

[starstryder]

check that whatever log file is given in the settings file can actually be written too and don't proceed if it's not.

[dpi209]

PHP is actually quite good at making this non-fatal - if you ini_set a log file that isn't writeable or doesn't exist, it falls back to the default SAPI log location (for Apache: the server-wide error_log). Since we aready checked that $BASE_DIR exists, we have following options:

a) Create the folder "logs" after we write the config file and be done with it or
b) Add a tab to the installer, where the user can check (checkbox) whether to log or not and check (radio) either logfile and add a location for said file, use the default SAPI log file or use syslog/eventlog
c) implement a and track b as an improvement.

I'm okay with either.

[starstryder]

This is a double sided issue: Want people to know where they are writing errors. Let's leave this open with the discussion until the code can shout at people accordingly. Admin panel or somewhere else.