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

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>
Signed-off-by: David Goulet <dgoulet@efficios.com>