Fail gracefully if performance metrics are incomplete

[christian-bromann]

Problem

We sometimes can not capture the performance of a page load due to problems either with Chrome or our internal drivers. If someone uses Speedo to analyze the results of such borked job he will get an error message like

Couldn't fetch performance results: TypeError: Cannot read property 'estimatedInputLatency' of undefined

We should let the command fail more gracefully and provide a better error message.