test-browser: improve unhandled error debugging

[alxndrsn]

master

Unhandled error in test page: {"name":"Error","message":""}

This PR

Unhandled error in test page: Unhandled Promise Rejection: {"name":"Error","message":""}
    at unknown (http://127.0.0.1:8000/packages/node_modules/pouchdb/dist/pouchdb.min.js:7:4434)
    at unknown (http://127.0.0.1:8000/packages/node_modules/pouchdb/dist/pouchdb.min.js:7:141513)

[SourceR85]

Just changed my mind: It's OK for me.

In case of old environments, console.log(' cause:', err.cause) will print

  cause: undefined

Everyone who sees this will come to the conclusion, that the environment is to old for the cause property.