Bug-2234: Propagate PCEP errors via RPC

-errors are added also in case the error was detected before request was sent

-add-lsp:
	-"SYMBOLIC-PATH-NAME in use" if such LSP already exists
	-"Capability not supported" if PCE does not have initiate capability
-update-lsp:
	-"unknown PLSP-ID" if such LSP does not exist
-remove-lsp:
	-"unknown PLSP-ID" if such LSP does not exist

Change-Id: Icb7096ef4bcfc5383751993fcc588168895a1ca6
Signed-off-by: Milos Fabian <milfabia@cisco.com>