split ProcessStatus into three variants

[bartlomieju]

No description provided.

[ry]

[12:02, 4/28/2020] Bartek:

error TS2339: Property 'code' does not exist on type '{ success: false; code: number; } | { success: false; signal: number; }'.
  Property 'code' does not exist on type '{ success: false; signal: number; }'.
    assert(status.code);
                  ~~~~
    at file:///Users/biwanczuk/dev/deno/cli/js/tests/process_test.ts:42:19

adding assertions doesn't really help
[12:03, 4/28/2020] Bartek: I'm not sure this change is good