Executes a given command on the server (the SITE EXEC command is added by the method).
Parameter
string $command - The command to be executed (without SITE EXEC).
Return value
mixed - true on success, otherwise PEAR::Error.
Throws
The returned PEAR_Error object in case of an error
is specific. There is only one errornumber which may occur. The message given by this error
will contain the directory where the error occurred in "Execution of command '$command' failed".
operation fails.
