[lit] Improve readability of failing scripts.

 - This only applies to scripts executed by the _internal_ shell script
   interpreter.

 - This patch reworks the log to look more like a shell transcript, and be less
   verbose (but in the interest of calling attention to the important parts).

Here is an example of the new format, for commands with/without failures and
with/without output:
```
$ true
$ echo hi
hi

$ false
note: command had no output on stdout or stderr
error: command failed with exit status 1

```

llvm-svn: 271610