ASSERTION_FAILED message result

[zyla]

Currently, message sent to a server can either be handled successfully or wait for state change.
Another option should be possible: terminate the program with error.
Example: reading from a buffer when it's empty, putting value into it when it's full.

Proposed syntax:
{ message | cond } -> ASSERTION_FAILED

[hator]

Or maybe change this to be aligned with the new syntax more (#5, #7) -- could look like this:

... -> { fail; }