Make entering a Login URI in the grid manager more robust.

Before, when anything but the exact correct Login URI was entered, the
user would either get no error, or a pop up saying "Cannot retrieve grid
info from server."

With this patch, that pop up includes what went wrong and stresses that
they check if they entered the correct Login URI and where to find it.

I was motivated to write this after looking at crash report 8405 where
a user who had downloaded Singularity for the very first time crashed
twice when entering things like "aviworlds" and " aviworlds.com:8002"
(not the space), and "http:aviworlds.com:8002" (missing '//').

Especially in the light of FS accepting Login URI without the 'http://',
a lot of grid websites (like that of aviworlds) might instruct users
to enter the url without http://. Ie, this user was told to enter
just "aviworlds.com:8002", and when that failed was instructed to
add 'http' in front of it...

Now the code accepts also really WEIRD things, but will never mess
up a good entry. For example, if you enter "pssshht:aviworlds.com:8002"
then that will work, as a side effect. The real objective however is
of course to let things work like: "aviworlds.com:8002  ",
"http:aviworlds.com:8002", " http:/aviworlds.com:8002", and to give
a usable error message when there is a typo in the hostname (Cannot
resolve hostname) or they forget to add a port number (404, or
connection refused), plus the text "Make sure you entered the correct
Login URI. An example of a Login URI is: \"http://cool.grid.com:8002/\",
this url can usually be found on the website of the grid."