CI: Show ipython directive errors
Current output of ipython directive errors in CI is essentially a blank line, so adding a bit more to try to show what error is happening.
```
(pandas-dev) ➜  ~ grep -B1 "^<<<-------------------------------------------------------------------------$" sphinx.log

<<<-------------------------------------------------------------------------
```
vs.
```
(pandas-dev) ➜  ~ grep -B10 "^<<<-------------------------------------------------------------------------$" sphinx.log


>>>-------------------------------------------------------------------------
Exception in /Users/danielsaxton/pandas/doc/source/user_guide/enhancingperf.rst at block ending on line 834
Specify :okexcept: as an option in the ipython:: block to suppress this message
  File "<ipython-input-66-d3ab2b9618fd>", line 1
    df.query("strings == "a" and nums == 1")
                          ^
SyntaxError: invalid syntax

<<<-------------------------------------------------------------------------
```
ok so this is a go-forward better error message? if that is the case lgtm. > ok so this is a go-forward better error message? if that is the case lgtm.

Yeah, saw a PR that failed this check but had to do a lot more digging to figure out what was wrong. I figure this makes it easier (ten lines is arbitrary, but should be fine usually). ok merge away