Add error variant name rule (#529)

* Add error variant name rule

* Adjust indentations for Rust code blocks

Author: Techassi

modules/contributor/pages/code-style-guide.adoc

@@ -246,25 +246,25 @@ The usage of `thiserror` is considered invalid.
 ----
 #[derive(thiserror::Error)]
 enum Error {
-  #[error("failed to read config file")]
-  FileRead(#[from] std::io::Error)
+    #[error("failed to read config file")]
+    FileRead(#[from] std::io::Error)
 }
 
 fn config_file(user: User) -> Result<(), Error> {
-  std::fs::read_to_string(user.file_path)?;
+    std::fs::read_to_string(user.file_path)?;
 }
 ----
 
 [source,rust]
 ----
 #[derive(Snafu)]
 enum Error {
-  #[snafu(context(false))]
-  FileRead { source: std::io::Error }
+    #[snafu(context(false))]
+    FileRead { source: std::io::Error }
 }
 
 fn config_file(user: User) -> Result<(), Error> {
-  std::fs::read_to_string(user.file_path)?;
+    std::fs::read_to_string(user.file_path)?;
 }
 ----
 
@@ -277,17 +277,54 @@ fn config_file(user: User) -> Result<(), Error> {
 ----
 #[derive(Snafu)]
 enum Error {
-  #[snafu(display("failed to read config file of user {user_name}"))]
-  FileRead {
-    source: std::io::Error,
-    user_name: String,
-  }
+    #[snafu(display("failed to read config file of user {user_name}"))]
+    FileRead {
+        source: std::io::Error,
+        user_name: String,
+    }
 }
 
 fn config_file(user: User) -> Result<(), Error> {
-  std::fs::read_to_string(user.file_path).context(FileReadSnafu {
-    user_name: user.name,
-  });
+    std::fs::read_to_string(user.file_path).context(FileReadSnafu {
+        user_name: user.name,
+    });
+}
+----
+
+====
+
+=== Error variant names
+
+All error variants must not include any unnesecarry prefixes or suffixes.
+Examples of such prefixes include (but are not limited to) `FailedTo` and `UnableTo`.
+Furthermore, examples for suffixes are `Error` or `Snafu` 
+Error variant names must however include verbs or identifiers as a prefix.
+
+[WARNING.code-rule,caption=Examples of incorrect code for this rule]
+====
+
+[source,rust]
+----
+#[derive(Snafu)]
+enum Error {
+    FailedToParseConfig,
+    HttpRequestError,
+    ConfigRead,
+}
+----
+
+====
+
+[TIP.code-rule,caption=Examples of correct code for this rule]
+====
+
+[source,rust]
+----
+#[derive(Snafu)]
+enum Error {
+    ParseConfig,
+    HttpRequest,
+    ReadConfig,
 }
 ----
 
@@ -305,14 +342,14 @@ It is recommended to start the error messages with "failed to..." or "unable to
 ----
 #[derive(Snafu)]
 enum Error {
-  #[snafu(display("Foo happened."))]
-  Foo,
+    #[snafu(display("Foo happened."))]
+    Foo,
 
-  #[snafu(display("Bar encountered"))]
-  Bar,
+    #[snafu(display("Bar encountered"))]
+    Bar,
 
-  #[snafu(display("arghh baz."))]
-  Baz,
+    #[snafu(display("arghh baz."))]
+    Baz,
 }
 ----
 
@@ -325,11 +362,11 @@ enum Error {
 ----
 #[derive(Snafu)]
 enum Error {
-  #[snafu(display("failed to foo"))]
-  Foo,
+    #[snafu(display("failed to foo"))]
+    Foo,
 
-  #[snafu(display("unable to bar"))]
-  Bar,
+    #[snafu(display("unable to bar"))]
+    Bar,
 }
 ----