currently, the `:st <n>` command in the debugger will simply silently
fail if the argument cannot be converted to an integer, or if it falls
outside the range of valid stack indices. this isn't too big of problem,
but it can be nicer to tell the user something went wrong, rather than
not give them any output and having them guess (esp. in the second case).
this commit adds two errors, one for each case:
1. the argument is not actually an integer, or is outside INT_MIN/MAX
-> "argument '%arg' is not a valid integer"
2. the argument is an integer outside the range of stack traces
-> "stack index must be between 0 and %max_frame, but was %arg"
Change-Id: I8109feeede79a9ad3db9ee7dc95d37e7dd19741a
