single bad value may cause datatype to become object
If reading a CSV, where one value got corrupt and doesn't remain a float, the inferred data type from read_csv would be object.
eg.
CSV contents

```
a,b
1.1,3.1
3.4,4,5
5.6,6.2
c3,7.2    

In [37]: df=pd.read_csv('test_bad.csv',error_bad_lines=False)
Skipping line 3: expected 2 fields, saw 3

In [38]: df
Out[38]: 
     a    b
0  1.1  3.1
1  5.6  6.2
2   c3  7.2
```

Several ways to look at this:
- read_csv would allow for explicitly giving column types. Those rows which do not adhere are dropped
- after seeing the warning, if the user can omit those rows using a simple method, rather than manually doing the same
