Datetime error message (unable to continue that PR)
- [ ] closes #10720
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Tests passed locally, but failed in Ci.
On some platforms
1) There's extra **%s** flag:
`AssertionError: Pattern "Unknown string format: Month 1, 1999. You can coerce to NaT by passing errors='coerce'" not found in "Unknown string format: %s Month 1, 1999. You can coerce to NaT by passing errors='coerce'"
`
2) or date is missing at all:
`AssertionError: Pattern "Unknown string format: Month 1, 1999. You can coerce to NaT by passing errors='coerce'" not found in "Unknown string format. You can coerce to NaT by passing errors='coerce'"`
Trying to figure it out... `str(e)` doesn't pass even locally. Output looks like: 
`"('Unknown string format:', 'Month 1, 1999'). You can coerce to NaT by passing errors='coerce'"`
Tried to use list comps. Doesn't help too. ping @WillAyd  ping №2: @jbrockmendel @baevpetr ci is not passing