Make the failure case of ToolkitRegistry.call more useful

[mattwthompson]

Right now, ToolkitRegistry.call tries the requested method in each of its wrapped toolkits and if they all fail, it raises a ValueError that squishes together the messages in each exception:

openff-toolkit/openff/toolkit/utils/toolkit_registry.py

Lines 376 to 380 in 7a3fad5

	msg += "Available toolkits are: {}\n".format(self.registered_toolkits)
	# Append information about toolkits that implemented the method, but could not handle the provided parameters
	for toolkit, error in errors:
	msg += " {} {} : {}\n".format(toolkit, type(error), error)
	raise ValueError(msg)

A small improvement would involve raising a custom exception instead of ValueError, using mostly the same message.

cc: @j-wags who can provide more thoughts on other solutions and possibly if it's worth refactoring the design in general.