worker/server: set a job error when heartbeat gets missing

Previously, we just used an empty struct when heartbeat failed. This is fine
for the osbuild job because it's treated as a failed one when
result.OSBuildResult == false which is the default value.

koji-finalize works differently though: It's in a failed state if there's
an job error of kojiError != "". So when failed heartbeat set the struct to
be empty, this was treated as success because there's no error.

Let's fix this by introducing a new error for the situation where we don't get
a heartbeat in time for a specific job.

internal/worker/clienterrors/errors.go

@@ -27,6 +27,7 @@ const (
 	ErrorEmptyPackageSpecs    ClientErrorCode = 24
 	ErrorDNFRepoError         ClientErrorCode = 25
 	ErrorJobDependency        ClientErrorCode = 26
+	ErrorJobMissingHeartbeat  ClientErrorCode = 27
 )
 
 type ClientErrorCode int