Validate Timestamp with invalid offset raises ValueError
- [x] closes #11630
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13116?src=pr) is **84.14%**

> Merging [#13116](https://codecov.io/gh/pydata/pandas/pull/13116?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #13116   diff @@
==========================================
  Files           137        137          
  Lines         50287      50287          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42311      42312     +1   
+ Misses         7976       7975     -1   
  Partials          0          0          
```
1. File `...das/tseries/index.py` (not in diff) was modified. [more](https://codecov.io/gh/pydata/pandas/commit/eef9274d0f9cbe9b7e720a0cc42d6cae4d4caa97/changes?src=pr#70616E6461732F747365726965732F696E6465782E7079) 
   - Misses `-1` 
   - Partials `0` 
   - Hits `+1`

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [af7bdd3...eef9274](https://codecov.io/gh/pydata/pandas/compare/af7bdd3883c8d61e9d9388d3aa699930eee7fff8...eef9274d0f9cbe9b7e720a0cc42d6cae4d4caa97)
 @gliptak The test you added is failing (see the travis logs)

BTW, you have a large diff, but I don't think you actually changed much? Please only change code that is actually related to the issue, and do not move around other code (difficult to review the diff)
 I think #12482 is in a better place to address that issue.
