Introduce generic ASAB error class

[byewokko]

• Introduce generic exceptions.ASABError, which follows the structure outlined by Elena.
• ASABError can be subclassed by different applications for their custom purposes.
• Introduce asab.web.rest.json.json_error_response function, which generates a HTTP JSON response from ASABError.
• Add examples.
• Add documentation.

async def my_handler(request):
  try:
    await create_something()
  except ASABError as e:
    L.log(asab.LOG_NOTICE, e.TechMessage)
    return asab.web.rest.json.json_error_response(request, e, status=409)

async def create_something():
  # Oops...
  raise ASABError(
    error_code=1000,
    tech_message="A record for {!r} already exists".format("stuff"),
    error_i18n_key="Unable to create '{{what}}', because it has already been created by '{{user}}'.",
    error_dict={"what": "stuff", "user": "the queen"},
  )

[byewokko]

not desirable, apparently