fix to_datetime default error suppression
- its not nice to change kwarg names without a deprecation cycle.
- why do you think this is a good idea?
- I think you could only realistically do this with an option that you could optionally set,
  e.g. change the kwarg to `None` then if its `None` use the default value of the option (I am talking
  about `pd.set_option(....)` here)
- this would need a fair amount of tests for this change
 I can see how switching to a boolean can break things, but ignoring errors by default is quite bad. How about I start by just changing the default?
 @cowpig well, the problem is `pd.to_datetime` will not barf on valid but non-parseable datelikes now.

eg. 

```
In [27]: pd.to_datetime('19MAY11',errors='raise')
TypeError: 'NoneType' object is not iterable

In [28]: pd.to_datetime('19MAY11',errors='raise',coerce=True)
Out[28]: NaT

In [29]: pd.to_datetime('19MAY11',errors='raise',format='%d%b%y')
Out[29]: Timestamp('2011-05-19 00:00:00')
```

So that error message is pretty bad. If you chane the default, then see what error message happens, and could think about doing that. I am not averse, its just that this affects lots of things possibly in subtle way.
 I think it belongs here:

date format `"%Y%m%d"` does not raise errors even when `errors='raise'` and silently returns `datetime.datetime` for out of bounds timestamp:

```
>>> pd.to_datetime('1300-01-01', format='%Y-%m-%d', errors='raise')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users//.virtual_envs/system/lib/python2.7/site-packages/pandas/tseries/tools.py", line 318, in to_datetime
    return _convert_listlike(np.array([ arg ]), box, format)[0]
  File "/Users//.virtual_envs/system/lib/python2.7/site-packages/pandas/tseries/tools.py", line 306, in _convert_listlike
    raise e
pandas.tslib.OutOfBoundsDatetime: Out of bounds nanosecond timestamp: 1300-01-01 00:00:00
>>> pd.to_datetime('13000101', format='%Y%m%d', errors='raise')
datetime.datetime(1300, 1, 1, 0, 0)
```
 closing pls reopen if/when updated
