Exception is triggered by backtesting 20210301-20210501 range with BAKE/USDT pair (binance). Pair data starts on 2021-04-30 12:00:00 and after adjusting for startup candles pair dataframe is empty.
Solution: Since there are other pairs with enough data - skip pairs with no data and issue a warning.
Exception:
```
Traceback (most recent call last):
File "/home/rk/src/freqtrade/freqtrade/main.py", line 37, in main
return_code = args['func'](args)
File "/home/rk/src/freqtrade/freqtrade/commands/optimize_commands.py", line 53, in start_backtesting
backtesting.start()
File "/home/rk/src/freqtrade/freqtrade/optimize/backtesting.py", line 502, in start
min_date, max_date = self.backtest_one_strategy(strat, data, timerange)
File "/home/rk/src/freqtrade/freqtrade/optimize/backtesting.py", line 474, in backtest_one_strategy
results = self.backtest(
File "/home/rk/src/freqtrade/freqtrade/optimize/backtesting.py", line 365, in backtest
data: Dict = self._get_ohlcv_as_lists(processed)
File "/home/rk/src/freqtrade/freqtrade/optimize/backtesting.py", line 199, in _get_ohlcv_as_lists
pair_data.loc[:, 'buy'] = 0 # cleanup from previous run
File "/home/rk/src/freqtrade/venv/lib/python3.9/site-packages/pandas/core/indexing.py", line 692, in __setitem__
iloc._setitem_with_indexer(indexer, value, self.name)
File "/home/rk/src/freqtrade/venv/lib/python3.9/site-packages/pandas/core/indexing.py", line 1587, in _setitem_with_indexer
raise ValueError(
ValueError: cannot set a frame with no defined index and a scalar
```
* Fix custom_sell() example to use rsi from last-available instead of trade-open candle, add a pointer to "Dataframe access" section for more info.
* Simplify "Custom stoploss using an indicator from dataframe example" greatly, add a pointer to "Dataframe access" section for more info.
Update custom_sell() example to comment that the current trade row is at trade open as written. Change "abstain" to something clearer for non-fluent English speakers.
