check if kauth.pub.js exists

[yukinying]

The ./do.sh config [host] would fail if kauth.pub.js does not exist, as it will be translate to:

   sed -i.bak 's/\[.*\]//' src/javascript/crypto/e2e/extension/config.js

which generates the following error when compiled

.src/javascript/crypto/e2e/extension/config.js:26: ERROR - Parse error. primary expression expected
  KAUTH_PUB: ,

1 error(s), 0 warning(s)

[maditya]

:+1: