libsubprocess: fail with ESRCH not ENOENT

Problem: when the subprocess server cannot find a process
by pid, it fails with ENOENT (No such file or directory).

Change to ESRCH (No such process).

Author: garlick

src/common/libsubprocess/server.c

@@ -102,7 +102,7 @@ static flux_subprocess_t *proc_find_bypid (subprocess_server_t *s, pid_t pid)
             return p;
         p = zlistx_next (s->subprocesses);
     }
-    errno = ENOENT;
+    errno = ESRCH;
     return NULL;
 }