refactor error API: update docs

Author: biojppm

changelog/current.md

@@ -1,18 +1,23 @@
 ### Improvements
 
-- [PR#503](https://github.com/biojppm/rapidyaml/pull/503) (fixes ): change error callbacks. Errors in ryml now have one of these types:
-  - parse error: when parsing YAML/JSON. See: `pfn_error_parse`,  `ErrorDataParse`, `ExceptionParse`, `err_parse_format()`.
-  - visit error: when visiting a tree (reading or writing). See: `pfn_error_visit`,  `ErrorDataVisit`, `ExceptionVisit`, `err_visit_format()`.
-  - basic error: other, non specific errors. See: `pfn_error_basic`,  `ErrorDataBasic`, `ExceptionBasic`, `err_basic_format()`.
-  - parse and visit errors/exceptions can be treated/caught as basic errors/exceptions
+- [PR#503](https://github.com/biojppm/rapidyaml/pull/503) (fixes [#399](https://github.com/biojppm/rapidyaml/issues/399)): change error callbacks.
+  - Errors in ryml now have one of these types:
+    - parse error: when parsing YAML/JSON. See: `pfn_error_parse`,  `ErrorDataParse`, `ExceptionParse`, `err_parse_format()`, `sample_error_parse`.
+    - visit error: when visiting a tree (reading or writing). See: `pfn_error_visit`,  `ErrorDataVisit`, `ExceptionVisit`, `err_visit_format()`, `sample_error_visit`.
+    - basic error: other, non specific errors. See: `pfn_error_basic`,  `ErrorDataBasic`, `ExceptionBasic`, `err_basic_format()`, `sample_error_basic`.
+    - parse and visit errors/exceptions can be treated/caught as basic errors/exceptions
   - Add message formatting functions to simplify implementation of error callbacks:
-    - `err_parse_format()`
-    - `err_visit_format()`
-    - `err_basic_format()`
-    - `location_format()`
-    - `location_format_with_context()`: useful to create a rich error message showing the region causing the error. 
-  - The functions above receive are generic, and can be used to a function 
-  - `Location`: 
+    - `err_parse_format()`: format/print a full error message for a parse error
+    - `err_visit_format()`: format/print a full error message for a visit error
+    - `err_basic_format()`: format/print a full error message for a basic error
+    - `location_format()`: format/print a location
+    - `location_format_with_context()`: useful to create a rich error message showing the YAML region causing the error, maybe even for a visit error if the source is kept and locations are enabled. 
+  - See the new header `c4/yml/error.hpp` (and `c4/yml/error.def.hpp` for definitions of the functions in `c4/yml/error.hpp`)
+  - See the relevant sample functions in the quickstart sample: `sample_error_basic`, `sample_error_parse` and `sample_error_visit`.
+  - There are breaking user-facing changes in the `Callbacks` structure:
+    - Removed member `m_error `
+    - Added members `m_error_basic`, `m_error_parse`, `m_error_visit`
+    - Added methods `.set_error_basic()`, `.set_error_parse()` and `.set_error_visit()`.
 
 
 ### Fixes