test: errors: doc updates

hledger/test/errors/README.md

@@ -49,7 +49,7 @@ Key:
 ## Current journal errors
 
 <!-- to update: erase the below then C-u M-! ./showall -->
-hledger 1.25.99-ge6bf04fce-20220316 error messages, last updated 2022-03-18:
+hledger 1.25, last updated 2022-03-18:
 
 ### parseable
 ```
@@ -176,3 +176,23 @@ seen in "a:c" in transaction at: /Users/simon/src/hledger/hledger/test/errors/./
 
 ```
 
+
+
+## Standard error format
+
+The proposed new format from [#1436][]. This is similar to megaparsec's pretty error output.
+
+```
+Error: [ID] FILE:LOCATION
+EXCERPT
+SUMMARY
+[DETAILS]
+```
+
+- begins with the word "Error"
+- ID is an optional error id, eg `HL1001` (in brackets ?). We might adopt these, similar to ShellCheck.
+- FILE is the file path.
+- LOCATION is `LINE[-ENDLINE][:COLUMN[-ENDCOLUMN]]					`
+- EXCERPT is a short visual snippet whenever possible, with the error region highlighted, line numbers, and colour when supported. This section must be easy for flycheck to ignore.
+- SUMMARY is a one line description/explanation of the problem. Currently we include contextual data in these for clarity, but it might be advantageous to use unchanging standard text.
+- DETAILS is optional additional details/advice when needed.