feat: error keyword

crates/aiken-lang/src/parser/lexer.rs

@@ -95,6 +95,7 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
 
     let keyword = text::ident().map(|s: String| match s.as_str() {
         "trace" => Token::Trace,
+        "error" => Token::ErrorTerm,
         "as" => Token::As,
         "assert" => Token::Assert,
         "check" => Token::Assert,

crates/aiken-lang/src/parser/token.rs

@@ -75,6 +75,7 @@ pub enum Token {
     Type,
     When,
     Trace,
+    ErrorTerm,
 }
 
 impl fmt::Display for Token {
@@ -154,6 +155,7 @@ impl fmt::Display for Token {
             Token::Trace => "trace",
             Token::Type => "type",
             Token::Test => "test",
+            Token::ErrorTerm => "error",
         };
         write!(f, "\"{}\"", s)
     }