support compilation with exception disabled

[hongxuchen]

Given failwith is the only function incompatible when -fno-exceptions is specified, I think it would be great to provide a version where exceptions are disabled. The compilation condition can be guarded by a macro guard.

[mayah]

To achieve that,

• we have to change the interface to return an error
• or abort immediately in failwith.

Actually I'm also considering an exception is harmful, but I'm not sure it's a good decision for now.

[hongxuchen]

Personally I prefer the immediate abortion since for now it introduces the minimal changes.

[mayah]

Now you can define your own failwith.