dummy gprs connect with error handling

src/modem.rs

@@ -1,3 +1,29 @@
+use std;
+
 pub struct Modem {
-    // TODO: connect to GPRS
+    is_connected: bool,
+}
+
+#[derive(Debug, Clone)]
+pub struct ModemConnectionError;
+
+impl std::fmt::Display for ModemConnectionError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "gprs connection error")
+    }
+}
+
+type ModemConnection = Result<Modem, ModemConnectionError>;
+
+impl Modem {
+    pub fn new() -> Self {
+        Self {
+            is_connected: false,
+        }
+    }
+    pub fn connect_to_gprs_ap(mut self, apn: &str, username: &str, password: &str)-> ModemConnection {
+        // TODO: set AT command for connecting to gprs
+        self.is_connected = true;
+        Ok(self)
+    }
 }