Preserve original remote error attributes in API exception

[knaperek]

What this fixes:

• wild "except" statement
• non-Pythonic direct type comparison (even worse, with "is")
• textual FB response message is parsed and preserved; this may be handy when trying to automatically propagate a meaningful textual error reason somewhere upstream in the code (e.g. operator dashboard panel)
• original HTTP status code is now preserved for easier debugging under the http_code attribute
• original requests status exception is nicely chained (with the extended raise from statement)