Histogram endpoint: review error responses in invalid scenarios

[sbesson]

The histogram endpoint functionality introduced in #125 returns a functional response in most scenarios. A few edge cases should be reviewed and improved:

• when channelId or z/t query parameters outside the image dimensions, the micro-service currently returns 200 with an informative error message
• when the histogram of a large image with missing pyramidal levels is requested, the micro-service currently returns 200 with an empty JSON

Similarly to #122, all these should probably return a 400 with an appropriate error message.

Originally posted by @sbesson in #125 (review)