BUG: freqstr may be parsed incorrectly
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Invalid freq like below doesn'traise `ValueError` properly. The PR makes all the below raise.

```
pd.tseries.frequencies.to_offset('U1')
# <Micro>
pd.tseries.frequencies.to_offset('3U1')
# <3 * Micros>
pd.tseries.frequencies.to_offset('-33-22U')
# <-22 * Micros>
```
