Warn that wildcards must be used as the last character in a string

Wildcards are currently only supported as the last character
in a string literal used in an event filter. This rule is not
checked which may confuse users who expect complete wildcard
support in filter expressions.

An error is now reported and the filter string is treated
as invalid.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>