Add more interesting compilation error messages

reid_src/test.reid

@@ -1,2 +1 @@
-let otus = "gotus";
+let otus = "dotus";
-let dotus = otus;

src/errors.rs

@@ -75,3 +75,24 @@ impl CompilerError {
         }
     }
 }
+impl Display for CompilerError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let text = match self {
+            CompilerError::Fatal => "Fatal error".to_string(),
+            CompilerError::VariableExists(pos, name) => {
+                format!("Variable '{}' already exists, re-assign at {}", pos, name)
+            }
+            CompilerError::VariableNotExists(pos, name) => {
+                format!("Variable '{}' does not exist, at {}", pos, name)
+            }
+            CompilerError::InvalidScopeExit(pos) => {
+                format!("Attempted to escape a scope invalidly at {}", pos)
+            }
+            CompilerError::LetFailed(pos, error) => {
+                format!("Let statement failed at {}:\n  {}", pos, error)
+            }
+            CompilerError::CanNotAssignVoidType => format!("Can not assign void type to variable"),
+        };
+        write!(f, "{}", text)
+    }
+}

src/main.rs

@@ -20,7 +20,7 @@ fn main() {
             dbg!(&parsed);
             let compiled = Compiler::from(parsed).compile();
             if let Err(error) = compiled {
-                eprintln!("Compilation error: {:#?}", error);
+                eprintln!("Compilation error: {}", error);
                 std::process::exit(1);
             }
             dbg!(compiled);