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ARROW-16431: [C++][Python] Improve AppendRowGroups error when schemas differ (#14029)

Fix [ARROW-16431](https://issues.apache.org/jira/browse/ARROW-16431)

Feel free to opine on specific error messages or the implementation as a whole. 👌 

Examples

```python
# meta1 and meta2 differ in column types
meta1.append_row_groups(meta2)
*** RuntimeError: AppendRowGroups requires equal schemas.
The two columns with index 0 differ.
column descriptor = {
  name: col1,
  path: col1,
  physical_type: INT64,
  converted_type: NONE,
  logical_type: None,
  max_definition_level: 1,
  max_repetition_level: 0,
}
column descriptor = {
  name: col2,
  path: col2,
  physical_type: INT64,
  converted_type: NONE,
  logical_type: None,
  max_definition_level: 1,
  max_repetition_level: 0,
}


# meta1 and meta2 differ in number of columns
meta1.append_row_groups(meta2)
*** RuntimeError: This schema has 2 columns, other has 1
```

Authored-by: Miles Granger <miles59923@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>