Handle JSON parse error on multiple build stream data chunks

[bryanlovely]

Where the streamed data to docker.buildImage is large (for instance in a large app with 100+ modules), a single message can be split among multiple chunks, in which case JSON.parse(data) will throw an error and the build will fail out.

This change temporarily adds the contents of data into a variable cumulativeData. If JSON.parse(cumulativeData) fails, it will just go on to the next chunk, add it to cumulativeData and try again. If it succeeds, it will send the parsed data to the log and clear cumulativeData until the next chunk.

Also, just to be sure nothing hangs around, cumulativeData is cleared on both error and end states of the stream.