chore: Add tests for errors

In order to better cover the expected bevahior of errors, this commit
adds a few tests to exercise the crate errors we use.

src/error.rs

@@ -1,5 +1,6 @@
 #![deny(missing_docs)]
 #[derive(thiserror::Error, Debug)]
+#[repr(u8)]
 /// Enum to define all of the possible errors that can occur during normal
 /// use of Nysm.
 pub enum NysmError {
@@ -35,6 +36,78 @@ pub enum NysmError {
 
 impl PartialEq for NysmError {
   fn eq(&self, other: &Self) -> bool {
+    println!("LEFT = {:?}", std::mem::discriminant(self));
+    println!("RIGHT = {:?}", std::mem::discriminant(other));
     std::mem::discriminant(self) == std::mem::discriminant(other)
   }
 }
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+  use serde::de::Error;
+
+  #[test]
+  fn test_serde_json_error() {
+    let error = NysmError::SerdeJson(serde_json::Error::custom("custom error"));
+    assert_eq!(error.to_string(), "Unable to parse data as json");
+  }
+
+  #[test]
+  fn test_serde_yaml_error() {
+    let error = NysmError::SerdeYaml(serde_yaml::Error::custom("custom error"));
+    assert_eq!(error.to_string(), "Unable to parse data as yaml");
+  }
+
+  #[test]
+  fn test_bat_print_error() {
+    let error = NysmError::BatPrint(Into::<bat::error::Error>::into("custom error"));
+    assert_eq!(error.to_string(), "Unable to pretty print data");
+  }
+
+  #[test]
+  fn test_io_error() {
+    let error = NysmError::IO(std::io::Error::new(
+      std::io::ErrorKind::Other,
+      "custom error",
+    ));
+    assert_eq!(
+      error.to_string(),
+      "Unable to read/write file caused by: custom error"
+    );
+  }
+
+  #[test]
+  fn test_aws_list_secrets_no_list_error() {
+    let error = NysmError::AwsListSecretsNoList;
+    assert_eq!(
+      error.to_string(),
+      "Unable to retrieve list of secrets from aws response"
+    );
+  }
+
+  #[test]
+  fn test_aws_secret_value_no_value_string_error() {
+    let error = NysmError::AwsSecretValueNoValueString;
+    assert_eq!(
+      error.to_string(),
+      "Unable to retrieve string value from aws response"
+    );
+  }
+
+  #[test]
+  fn test_aws_secret_value_update_error() {
+    let error = NysmError::AwsSecretValueUpdate;
+    assert_eq!(error.to_string(), "Unable to update secret value");
+  }
+
+  #[test]
+  fn test_partial_eq() {
+    let error1 = NysmError::SerdeJson(serde_json::Error::custom("custom error"));
+    let error2 = NysmError::SerdeJson(serde_json::Error::custom("custom error"));
+    let error3 = NysmError::SerdeYaml(serde_yaml::Error::custom("different error"));
+
+    assert_eq!(error1, error2);
+    assert_ne!(error1, error3);
+  }
+}