sweet: implement misc UX improvements

- Give a better error message when running sweet from somewhere other
  than x/benchmarks/sweet.
- Reference the actual flag names when referring to them in the run
  subcommand's error messages.
- Document where the sweet command expects to run from by default.
- Make -short imply -count=1 if -count is not set.
- Print the number of benchmark runs that will happen.
- Clean up the README a bit.

Change-Id: I5adc5af69b9f4e86e3a79f9bd2735353c6834e0b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/449296
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
2 files changed