catch and report the database error

[aammd]

Here is an example of a good error from the database:

Response [http://www.zoology.ubc.ca/~lui/v1/api/?route=matrix&action=list&dataset_id=96]
  Date: 2017-01-10 10:14
  Status: 412
  Content-Type: application/json; charset=utf-8
  Size: 48 B
{
	"success": false,
	"error": "NO_BROMELIADS"

however all we get from bwg_get() is

 Error in bwg_get("matrix", .) : Precondition Failed (HTTP 412).