GoogleVisionCache shall not store vision analysis result when result has had an error

[mklaehn]

Describe the bug
VisionAnalysisResult must never be store the ImageContentAnalysis in case the result contained an error. This may happen when Gloud Vision API eroors on loading the imageURL for example.

Should a processing error be cached then successive runs of the analysis will get the failed result from the cache instead of retriggering the request and getting a hopefully succesful request result.

Expected behavior
A failed image analysis result is either not stored in cache or should it be, then it will be rerequested upon the next retrieval of the analysis from the cache.