Notify the user if a filter is already enabled

When using the enable-event command in conjunction with the --filter
option, a user can specify a filter expression to refine the trace
output.

As stated in the lttng(1) man page, only the first activation of a
filter on an event will work. Subsequent activation of any filter
expression on the same event will fail.

e.g:

> lttng enable-event app:tp -s session -u --filter 'somefield > 42'

Case: invalid filter expression
> lttng enable-event app:tp -s session -u --filter 'invalid expression'
  Error: Error setting filter
  Warning: Some command(s) went wrong

> ...

Case: filter already enabled for event app:tp
> lttng enable-event app:tp -s session -u --filter 'someotherfield < 42'
  Error: Error setting filter
  Warning: Some command(s) went wrong

This commit differentiate the case where a filter was already set for
the specified event from the generic 'Error setting filter' error
message.

e.g:

> lttng enable-event app:tp -s session -u --filter 'somefield > 42'

Case: invalid filter expression
> lttng enable-event app:tp -s session -u --filter 'invalid expression'
  Error: Error setting filter
  Warning: Some command(s) went wrong

> ...

Case: filter already enabled for event app:tp
> lttng enable-event app:tp -s session -u --filter 'someotherfield < 42'
  Error: Filter on event app:tp is already enabled (channel 0, session session)
  Warning: Some command(s) went wrong

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>