DOC: Fix height of the comic strip + minor doc improvements

Author: kernc

backtesting/__init__.py

@@ -1,6 +1,6 @@
 """
 
-![xkcd.com/1570](https://imgs.xkcd.com/comics/engineer_syllogism.png)
+![xkcd.com/1570](https://imgs.xkcd.com/comics/engineer_syllogism.png){: height=263}
 
 ## Manuals
 

backtesting/backtesting.py

@@ -126,7 +126,7 @@ def init():
         try:
             value = func(*args, **kwargs)
         except Exception as e:
-            raise RuntimeError(f'Indicator "{name}" error') from e
+            raise RuntimeError(f'Indicator "{name}" error. See traceback above.') from e
 
         if isinstance(value, pd.DataFrame):
             value = value.values.T

backtesting/lib.py

@@ -278,10 +278,11 @@ def SMA(series, n):
     if func is None:
         def func(x, *_, **__):
             return x
+    assert callable(func), 'resample_apply(func=) must be callable'
 
     if not isinstance(series, (pd.Series, pd.DataFrame)):
         assert isinstance(series, _Array), \
-            'resample_apply() takes either a `pd.Series`, `pd.DataFrame`, ' \
+            'resample_apply(series=) must be `pd.Series`, `pd.DataFrame`, ' \
             'or a `Strategy.data.*` array'
         series = series.s