Only trap E_ERROR in session handling

[juliusknorr]

Otherwise PHP errors with any level would cause an ErrorException that would make the code behave as if the operation failed.

E.g. as reported in #28463 where the sesison gc failed which then leads to the session cookie being invalidated in

server/lib/private/Session/Internal.php

Lines 55 to 59 in c1ea6a8

	try {
	$this->startSession();
	} catch (\Exception$e) {
	setcookie($this->invoke('session_name'), '', -1, \OC::$WEBROOT ?: '/');
	}

See #28463 (comment) for more details.

[juliusknorr]

/backport to stable22

[juliusknorr]

/backport to stable21

[juliusknorr]

/backport to stable20