Add missing prompt when command doesn't exist (MI)

It fixes the following example:
```
$ bin/lldb-mi --interpreter
(gdb)
-not-found
^error,msg="Driver. Received command '-not-found'. It was not handled. Command 'not-found' not in Command Factory"
-not-found
^error,msg="Driver. Received command '-not-found'. It was not handled. Command 'not-found' not in Command Factory"
```

After the fix it looks like:
```
$ bin/lldb-mi --interpreter
-not-found
^error,msg="Driver. Received command '-not-found'. It was not handled. Command 'not-found' not in Command Factory"
(gdb)
-not-dounf
^error,msg="Driver. Received command '-not-dounf'. It was not handled. Command 'not-dounf' not in Command Factory"
(gdb)
```

llvm-svn: 229131