my.core.serialize: implement default flag

Lots of surrounding changes from this discussion:
0593c69056

renamed/split error_to_json into error_to_row, moved
pandas-aware functions to my.core.pandas, leaving
generic ones in my.core.error.

my/core/error.py

@@ -145,15 +145,9 @@ def extract_error_datetime(e: Exception) -> Optional[datetime]:
 
 import traceback
 from .common import Json
-def error_to_json(e: Exception, *, dt_col: str='dt', tz=None) -> Json:
-    edt = extract_error_datetime(e)
-    if edt is not None and edt.tzinfo is None and tz is not None:
-        edt = edt.replace(tzinfo=tz)
+def error_to_json(e: Exception) -> Json:
     estr = ''.join(traceback.format_exception(Exception, e, e.__traceback__))
-    return {
-        'error': estr,
-        dt_col : edt,
-    }
+    return {'error': estr}
 
 
 def test_datetime_errors() -> None: