Warn if error message doesn't conform to style guideline

What it does

According to the Rust API Guidelines:

The error message given by the Display representation of an error type should be lowercase without trailing punctuation, and typically concise.

Clippy could make sure that error message are lowercase without trailing punctuation, by checking

• the Display impl on types that have an Error impl
• .ok_or("error message"), .map_err(|_| "error message") and similar methods

Lint Name

misformatted_error_message

Category

style

Advantage

An error message that conforms to the official guidelines fits more easily into the ecosystem

Drawbacks

No response

Example

.ok_or_else(|| format!("Unknown wifescore or grade `{}`!", threshold_str))?

Could be written as:

.ok_or_else(|| format!("unknown wifescore or grade `{}`", threshold_str))?

Start with the Display impl and Error impl cases, then examine the ok_or, map_err, and ok_or_else examples described in the issue. Define the lint's supported error-message forms and verify that it identifies messages that are not lowercase or that have trailing punctuation, while accepting conforming messages.