Invoke on_error_handler as described in the docs

[dlenskiSB]

According to the example in the README, the on_error handler should be called with two parameters, of which the first is the CertStreamClient instance:

certstream-python/README.md

Lines 61 to 63 in 97eb5b3

	def on_error(instance, exception):
	# Instance is the CertStreamClient instance that barfed
	print("Exception in CertStreamClient! -> {}".format(exception))

[dlenskiSB]

In addition to aligning the docs with the code, this fix is needed to be able to construct a CertStreamClient object with an error handler that actually has access to the instance object.

Currently, I have to do this by modifying the .on_error_handler after construction:

from functools import partial

def on_error_handler(instance, exc):
    # ...

# FIXME: Docs say instance is passed as first param, but it isn't
c = CertStreamClient(callback, url='wss://certstream.calidog.io/', skip_heartbeats=True)
c.on_error_handler = partial(on_error_handler, c)