Remove generic error reporting from the lttng client

Unsuccessful command results are reported by the lttng client
which often results in strange error messages of the form:

Error: Could not rotate ...
Error: Command error

Considering that most commands correctly report their errors,
it is safe to remove those generic error reports.

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