foot/footclient: use a custom exit code when foot/footclient fail to run

Normally, foot and footclient uses the exit code from the client
application (i.e. the shell).

However, foot (or footclient) itself may fail to run; if run outside
of a Wayland session, or no fonts are installed, or the client
application/shell cannot be found (“foot lsdjfldsjf”) etc.

Up until now, there has been no way to differentiate these kind of
failures from the client application exiting with code 1.

This patch changes foot’s failure exit code to -27/229, and
footclient’s to -28/228. Note that footclient will exit with foot’s
-27/229 if footclient ran successfully, but the foot server failed to
instantiate a new window.

Closes #466.

CHANGELOG.md

@@ -51,6 +51,12 @@
 * Point values in `line-height`, `letter-spacing`,
   `horizontal-letter-offset` and `vertical-letter-offset` are now
   rounded, not truncated, when translated to pixel values.
+* Foot’s exit code is now -27/229 when foot itself failed to launch
+  (due to invalid command line options, client application/shell not
+  found etc). Footclient’s exit code is -28/228 when it itself fails
+  to launch (e.g. bad command line option) and -27/229 when the foot
+  server failed to instantiate a new window
+  (https://codeberg.org/dnkl/foot/issues/466).
 
 
 ### Deprecated