API: combine 'coerce' and 'errors' keywords of to_datetime 
From #10636 

When parsing an invalid date, there are some possible different actions to take:
- ignore -> just returning the original value unparsed (`errors='ignore'`)
- raise an error (`errors='raise'`)
- coerce / convert to NaN (`coerce=True`)

AFAIK, these three options are just three different options and mutually exclusive (or is there somewhere in the codebase an interaction between both?), so they could be controlled by a single keyword. 
Having two keyword can make it even confusing what you should expect when combining those two, eg the example of coerce=True and error=raise.

Proposal: add another `'coerce'` option to the `errors` keyword that is equivalent to `coerce=True`

-> this gives the `errors` keyword three options: `'ignore'|'raise'|'coerce'`
