tokeniser: add some actual tokeniser errors

TODO: Return these lol

assembler/src/source/tokeniser/error.rs

@@ -1,12 +1,33 @@
 //! This module contains the error types for the tokeniser.
 
 #[derive(Debug, Clone, Copy)]
-pub enum TokeniserError {}
+/// Types of errors that may be returned during tokenisation.
+pub enum TokeniserError {
+    /// An unexpected character was found in the source code.
+    UnexpectedChar(char),
+    /// An unterminated string literal was found. [`SourceInfo`] will be attached if this
+    /// was returned.
+    UnterminatedString,
+    /// An invalid number format was encountered when parsing a literal value
+    /// ([`TokenType::Immediate`]).
+    InvalidNumber(&'static str),
+    /// An unrecognized token was encountered.
+    UnrecognisedToken,
+}
 
 impl TokeniserError {}
 
 impl std::fmt::Display for TokeniserError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "TODO!!!!!!")
+        match self {
+            Self::UnexpectedChar(c) => write!(f, "unexpected char '{c}' found in input")?,
+            Self::InvalidNumber(lit) => {
+                write!(f, "invalid integer literal \"{lit}\" found in input")?;
+            }
+            &Self::UnrecognisedToken => write!(f, "unrecognised token found in input")?,
+            &Self::UnterminatedString => write!(f, "unterminated string literal")?,
+        }
+
+        Ok(())
     }
 }