Implement basic error handling

src/error.rs

@@ -1 +1,38 @@
+use crate::common::Pos;
+use std::error::Error;
+use std::fmt::{self, Display};
+use std::io;
 
+#[derive(Debug)]
+pub struct SassError {
+    message: String,
+    pos: Pos
+}
+
+impl SassError {
+    pub fn new<S: Into<String>>(message: S, pos: Pos) -> Self {
+        SassError { message: message.into(), pos }
+    }
+
+    pub fn unexpected_eof(pos: Pos) -> Self {
+        SassError { message: String::from("unexpected eof"), pos }
+    }
+}
+
+impl Display for SassError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "error: {} at {}", self.message, self.pos)
+    }
+}
+
+impl From<io::Error> for SassError {
+    fn from(error: io::Error) -> Self {
+        SassError{ pos: Pos::new(), message: format!("{}", error) }
+    }
+}
+
+impl Error for SassError {
+    fn description(&self) -> &'static str {
+        "SASS parsing error"
+    }
+}