BUG: suppress error raise by nonnumeric columns when plotting DataFrame ...
...#3108

suppressing AttributeError here so you can plot the DataFrame and automatically exclude non-numeric columns. @y-p @wesm @jreback any thoughts on 1) whether it should raise instead of exclude and 2) whether catching the AttributeError here is too dangerously broad?

maybe add an option `raise_on_error=True` as the default for plotting? (we use this option syntax else, mostly internally but use can set it)
 @jreback done. By default it will raise and the users should see a helpful error msg that includes a hint to set "raise_on_error=False" to suppress the exception.
 I guess the warning makes this sort of ok, but the exception is "AttributeError" not "ValueError" ,
it's a very round-about way to catch the fact that the user is trying to plot non-numerical data 
and I think it might prove brittle. 

+1 for catching it higher up the stack, rather the 50 fathoms down mpl with a seemingly 
unrelated error.
 why don't we do a get_numeric_data and just avoid this issue?
though I guess user will be wondering why there string data is not plotting
 Merge?
