Make NDT error messages more informative

[critzo]

Many users emailing support@measurementlab.net report confusing errors from NDT when it doesn't work for them. For example, if the user's connection is very slow, NDT can't measure the connection, but reports "NaN.undefined" as the speed to the user. In the details tab for the NDT client, this is reported as:

Error occured while performing test:
Test failed: 150

NDT client errors should be made more informative to the user.