Fixing set of firefox preferences for logging prefs.

After this fix fxdriver correctly set logging preferences.
Fix handling of conversion errors at node side.

It prevents 500 response with HTML body when
node failed at conversion of input json data.

1. move conversion into base try catch of request handler.
2. report better info wen invalid input is provided for setBrowserSize/setBrowserPosition.
3. make sure thet python binding sends integers as input for setBrowserSize/setBrowserPosition.
use check with wait for CrashWhenStartingBrowserTest.serverCrashesStartingFirefox
Allow to set default logLevel for standalone-server.

Now there are 3 variants possible:
1. (existed and unchanged) passing -debug parameter will set FINE level.
2. (new) passing no -debug and -Dselenium.LOGGER.level=<value>
will set <value> level.
3. (existed and unchanged) passing nothing will set INFO level.

Fixes issue 2285.