Recover from parse errors when parse_error() returns true (#3989)

The return value of json_sax::parse_error() was documented both as "must
return false" and as "whether the parsing should continue", and the code
just passed it on. For JSON text, parsing stopped anyway, but sax_parse()
could report success for invalid input. The binary readers read on after
the error, looping forever on a CBOR indefinite-length array without its
end.

Now false stops parsing, and true recovers from the error:

- JSON text is repaired with the smallest local edit (insert a missing
  ',' or ':', remove a stray token, keep the readable part of a broken
  string or number, null for a value that cannot be read, close the
  innermost container at a wrong closing bracket and all of them at the
  end of the input), and parsing continues. The SAX events stay balanced,
  every key is followed by exactly one value, and each token is reported
  at most once.
- The binary formats cannot resynchronize, so they stop, but complete
  the value read so far.

sax_parse() returns false after any error. parse(), accept(), and the
from_*() functions never recover and compile to the same code as before.

Supersedes #4522.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/json_sax/parse_error.md

@@ -21,7 +21,14 @@ A parse error occurred.
 
 ## Return value
 
-Whether parsing should proceed (**must return `#!cpp false`**).
+Whether to recover from the error:
+
+- `#!cpp false` stops parsing.
+- `#!cpp true` recovers from the error: JSON text is repaired and parsing continues; for the binary formats, the value
+  read so far is completed and parsing stops. See [error recovery](../../features/parsing/error_recovery.md) for how
+  errors are repaired.
+
+Either way, [`sax_parse`](../basic_json/sax_parse.md) returns `#!cpp false`.
 
 ## Examples
 
@@ -39,6 +46,22 @@ Whether parsing should proceed (**must return `#!cpp false`**).
     --8<-- "examples/sax_parse.output"
     ```
 
+??? example
+
+    The example below shows how a SAX parser recovers from errors.
+
+    ```cpp
+    --8<-- "examples/sax_parse__error_recovery.cpp"
+    ```
+    
+    Output:
+    
+    ```
+    --8<-- "examples/sax_parse__error_recovery.output"
+    ```
+
 ## Version history
 
 - Added in version 3.2.0.
+- Returning `#!cpp true` recovers from the error since version 3.13.0; before, parsing stopped, but the result of
+  [`sax_parse`](../basic_json/sax_parse.md) could be wrong.