[FLINK-17792][tests] Catch and log exception if jstack fails

jstack can fail if the JVM process that we want to sample exits while or
before we invoke jstack. Since a JVM process is free to exit at any
time, we should not propagate the exception so that we do not fail the
test prematurely.

flink-jepsen/src/jepsen/flink/utils.clj

@@ -133,7 +133,10 @@
 
 (defn- write-jstack!
  [pid out-path]
- (c/exec :jstack :-l pid :> out-path))
+ (try
+ (c/exec :jstack :-l pid :> out-path)
+ (catch Exception e
+ (warn e "Failed to invoke jstack on pid" pid))))
 
 (defn dump-jstack-by-pattern!
  "Dumps the output of jstack for all JVMs that match one of the specified patterns."