Add epc:start-epc-deferred
epcs.el must require cl at eval time
Test epc:start-epc-deferred
Add epc:demo-dir
Merge branch 'start-epc-deferred' of https://github.com/tkf/emacs-epc into tkf-start-epc-deferred
Fix failures of epc-test

Since the merge of kiwanami/emacs-deferred#7 changed the way of error handling
of the deferred framework, some tests failed.
This commit fixed these failures.
Now, the symbol `epc-error` has `error-conditions` property and
`error-message` one, so as to respond to the emacs error handling mechanism.