js: Error handling for browser errors

• Connection.js
  • Add some meaningful nativeMessaging feedback for failing communication due to misconfiguration or other browser-originated fails
  • add an "isDisconnected" property
  • "isNativeHostUnknown" tries to match browser's feedback string if the browser does not find gpgme-json
• init.js
  • initialization will now reject with a more meaningful error if the configuration is not set up or other browser-based errors (chrome.runtime.lastError) are present. This should speed up the normal initialization (not having to waiting for a timeout any more in case of improper setup)
• errors.js
  • CONN_NATIVEMESSAGE: New error that passes the browser's nativeMessaging error
  • CONN_NO_CONFIG: native messaging error indicating that the nativeMessaging host was not set up properly
• unittests.js:
  • added the "isDisconnected" property to the startup tests
  • added tests for proper behavior of connection checks