Fix checker crashes silently passing validation as valid

[RPRKLR]

The two catch blocks in checkBehaviorReport() assumed every caught
error was a custom {path, error} object (as thrown by
Behavior.createStructureInfo / checkStatemachine). A real JS exception
(e.g. a TypeError from a bug in the checker) has no .error property, so:

- checking.js:189 pushed `error.error` == undefined onto
  report.fatal_errors. checkBehavior() then returns fatal_errors[0]
  == undefined, which callers read as "no error" -> a behavior that
  actually failed the check could be saved or started as if valid.

- checking.js:199 called `error.path.replace(...)` unconditionally.
  On a real exception .path is undefined, so this threw a second
  TypeError inside the catch handler, masking the original error.

Both handlers now resolve a non-empty message
(error.error -> error.message -> String(error)), so a checker crash
always produces a real fatal error instead of undefined. The
container-navigation logic is guarded by if (error.path != undefined)
so it only runs for the custom {path, error} objects that carry a path.