Check for write access and display friendly alert

Players often have write-access problems because they put MUSHclient in a bad place where it can't write files like settings and state. Can MUSHclient do simple write checks before anything else and display a friendly message?

I just got a bug report about a cryptic I/O error because MUSHclient was not able to drop table control in MUSHclient.cpp::InitInstance()

( https://github.com/fiendish/aardwolfclientpackage/issues/203 )
(also http://www.mushclient.com/forum/?id=11189&reply=12 )

I believe that the sqlite3_open call in InitInstance will not return an error code as expected if the file exists but is read-only.

Added in version 4.92.

https://github.com/nickgammon/mushclient/commit/da7e6b1