chgserver: handle ParseError during validate
Currently the validate command in chgserver expects config can be loaded
without issues but the config can be broken and chg will print a stacktrace
instead of the parsing error, if a chg server is already running.
This patch adds a handler for ParseError in validate and a new instruction
"exit" to make the client exit without abortmsg. A test is also added to make
sure it will behave as expected.
