Expose error fields

[aljazerzen]

Currently, we do have property getters for error fields:

edgedb-python/edgedb/errors/_base.py

Lines 98 to 127 in ca40114

	@property
	def _position(self):
	# not a stable API method
	return int(self._read_str_field(FIELD_POSITION_START, -1))
	@property
	def _position_start(self):
	# not a stable API method
	return int(self._read_str_field(FIELD_CHARACTER_START, -1))
	@property
	def _position_end(self):
	# not a stable API method
	return int(self._read_str_field(FIELD_CHARACTER_END, -1))
	@property
	def _line(self):
	# not a stable API method
	return int(self._read_str_field(FIELD_LINE_START, -1))
	@property
	def _col(self):
	# not a stable API method
	return int(self._read_str_field(FIELD_COLUMN_START, -1))
	@property
	def _hint(self):
	# not a stable API method
	return self._read_str_field(FIELD_HINT)

... but they are not considered stable API. We should change that and also expose details field.

We do have similar code in the server:

https://github.com/edgedb/edgedb/blob/master/edb/errors/base.py#L178-L200

[msullivan]

I think probably details should be dumped in the default formatting