Use a custom enum instead of Result for `Pad::sticky_events_foreach()` / `BufferList::foreach()`

changed milestone to %0.18.0

mentioned in commit tpm/gstreamer-rs@33ee92ed

mentioned in merge request !897 (merged)

mentioned in commit tpm/gstreamer-rs@724aca79

mentioned in commit tpm/gstreamer-rs@c8b2ed42

mentioned in commit tpm/gstreamer-rs@8fa84683

mentioned in commit tpm/gstreamer-rs@c8c2ece0

mentioned in commit tpm/gstreamer-rs@adc323b6

mentioned in commit tpm/gstreamer-rs@662572f7

mentioned in commit tpm/gstreamer-rs@c5f687bd

mentioned in commit tpm/gstreamer-rs@8125bf85

mentioned in commit tpm/gstreamer-rs@393093d0

mentioned in commit tpm/gstreamer-rs@a9667657

mentioned in commit tpm/gstreamer-rs@cc6c4be8

mentioned in commit tpm/gstreamer-rs@74a763f9

mentioned in commit tpm/gstreamer-rs@e0e17b8b

closed with merge request !897 (merged)

closed with commit e0e17b8b