Supress errors about file not found

When we're waiting for browserstack to start, the log file that is
checked might not exist yet resulting in a "file not found error".

Adding '-s' to grep supress those errors.

One consern though is that if the file is missing because of other
problems, this might be hard to debug.

Maybe there stderr should be redirected to a file so it can be debugged.

Something like:
```
while ! grep -s "${DONE_MSG}" ${BROWSER_STACK_LOG_FILE} 2>
grep_errors.log > /dev/null; do
```

scripts/wait-browserstack.sh

@@ -19,11 +19,11 @@ if [ "${BROWSER_STACK_TUNNEL}" = "true" ]; then
   echo "Waiting for BrowserStack tunnel to start..."
   # Required params
   [ -z "${BROWSER_STACK_LOG_FILE}" ] && die "Required env var BROWSER_STACK_LOG_FILE"
-  while ! grep "${DONE_MSG}" ${BROWSER_STACK_LOG_FILE} >/dev/null; do
+  while ! grep -s "${DONE_MSG}" ${BROWSER_STACK_LOG_FILE} >/dev/null; do
     echo -n '.'
     sleep 0.2;
   done
   echo "BrowserStack tunnel started! (wait-browserstack.sh)"
 else
   echo "Won't start BrowserStack tunnel due to BROWSER_STACK_TUNNEL false"
-fi
+fi