Instantiate plugins AFTER connecting to the db.

This allows plugins to use the connection from their constructor if the
script created it, dpeending on $_no_db_connection global variable.
This revert commit b43084e022695d6b506799a1815fb4735bd57d13. I did a few
tests, mainly on intro page, but couldn't find the bug this commit tried
to fix...
plugin report: Remove non used global var in constructor.
Add try/catch blocks around plugin instanciation so they can throw an exception if needed.

This is usefull if a plugin don't want to start for some reasons.
Throwing an exception from the plugin constructors allows to remove this
plugin entirely during the script execution.
Instanciate $plugin_manager from login.php script if needed

This answers the question in commit message:
7d89d7cca40b5a8c3ac1874436630993f518410c. When the script require a
database connexion and the user must login, or in some cases of some
other related errors, we ar eincluding the login.php script and exit
immediatly. The login.php script requires the $plugin_manager, but it
is instanciated AFTER the connection codeblock (see commit 7d89d7cc),
leading to a FATAL error.