Check for file existance in file modification check

We sometimes start health checks before a file is actually written out
by the service. This results in a bash error in the logs. Let's check
the file exists before trying to use it in the health check.

Change-Id: I58815050117af20abc547f46d15073ac0bf8ca74
Closes-Bug: #1824246

healthcheck/common.sh

@@ -49,6 +49,11 @@ healthcheck_file_modification () {
     file_path=$1
     limit_seconds=$2
 
+    # if the file doesn't exist, return 1
+    if [ ! -f $file_path ]; then
+        echo "${file_path} does not exist for file modification check"
+        return 1
+    fi
     curr_time=$(date +%s)
     last_mod=$(stat -c '%Y' $file_path)
     limit_epoch=$(( curr_time-limit_seconds ))