Elektra has a lot of developers and hence the error messages vary in how they are written. This guideline should unify the message format so that users will get consistent error messages.

All error messages start with a capital letter. Reword sentences that theoretically require a lowercase first letter such as regex patterns, method names or variable strings ("%s"-strings) to have a normal word at the start of the sentence. Also surround method names with single quotes. Example: kdbSet() not supported should be changed to ‘Method 'kdbSet()’ is not supported`
Always report all information which could be useful for the user. For some error types, templates are provided to not forget something. (eg. keys, files, errno, etc.)
If the error reason is in beneath a variable string (such as errno or a caught exception), write "Reason: %s", variable_as_string at the end of the error message. Also end the preceding sentence with a dot. Example: "XY failed. Reason: %s", exception.what().
The last sentence (or single sentence of your message if you just provide one) must not end with a dot. This should encourage users to continue reading if other messages appear.
If your message has multiple sentences, separate them with dots and start with a capital letter like you would do in the normal English language.
Short but many sentences are preferable over long ones.
Use abbreviations and acronyms with care. Not everybody will know them. You can write them in brackets though. (eg. No Byte Order Mark (BOM) found instead of No BOM found. But XML does not support feature xy is fine).
Never use exclamation marks at the end of sentences.
If it is unclear where embedded strings in error messages start and end ("%s"-strings), surround them by single quotes '. Sentences containing a variable string and end with Reason: s do not need extra quotes because it is clear where they start and end.

Examples

Actual reason might be in errno or an exception

"The configuration file %s contains invalid syntax at line %s. Reason: %s", file.path(), line, e.what() Result: The configuration file /etc/conf/file.csv contains invalid syntax at line 6. Reason: Missing column
"Could not rename file %s. Reason: %s" file.path(), e.what() Result: Could not rename file /etc/conf/file.yml. Reason: File is not existent
‘"The key %s contained the value ’%s', but only 'unmounted' is supported for non-global clauses at the moment", keyName(key), keyString(key) Result:The key user:/my/key contained the value 'mounted', but only 'unmounted' is supported for non-global clauses at the moment`