Fix errcheck issues in server/channels/app/plugin_deadlock_test.go (#… #3

Job	Run time
master-ci / Check mocks	1m 56s
master-ci / Check app layers	20s
master-ci / Generate email templates	22s
master-ci / Check serialization methods for hot structs	17s
master-ci / Check go mod tidy	27s
master-ci / Check style	2m 18s
master-ci / Vet API	2m 39s
master-ci / Check migration files	8s
master-ci / Check mmctl docs	1m 35s
master-ci / Check store layers	18s
master-ci / golangci-lint	4m 4s
master-ci / MySQL to PostgreSQL Migration / MySQL -> Postgres Migration	5m 58s
master-ci / Build mattermost server app	1s
master-ci / Postgres with binary parameters	0s
master-ci / Run mmctl tests / mmctl	35m 30s
master-ci / MySQL / MySQL	1h 37m 28s
master-ci / Postgres / Postgres	1h 26m 54s
	4h 0m 15s

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