ledflasher: Add graceful error handling

ledflasher relied on weaved/libweave for command parameter validation
which is no longer provided by libweave, so it needs to be less strict
about accepting commands with invalid parameters. Instead of just crashing
the application should just abort the command with corresponding error
message.

BUG: 25829264
Change-Id: Iae09b0d68006a1d5226d15d8b40552d42c7ab411