Added error checking and derivedFrom on registers

Closes rust-embedded#18
Closes rust-embedded#21

Cargo.toml

@@ -10,3 +10,4 @@ version = "0.5.0"
 
 [dependencies]
 xmltree = "0.3.2"
+error-chain = "0.7.2"

generate-tests.sh

@@ -49,7 +49,7 @@ EOF
 fn $device() {
     let xml = include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/$device_path"));
 
-    svd::parse(xml);
+    svd::parse(xml).unwrap();
 }
 EOF
         done

src/errors.rs

@@ -0,0 +1,17 @@
+use xmltree;
+use std::str;
+use std::num;
+error_chain! {
+    foreign_links {
+        XmlParseError(xmltree::ParseError);
+        StrParseBoolError(str::ParseBoolError);
+        NumParseIntError(num::ParseIntError);
+
+    }
+    errors {
+        MissingMandatory(t: String) {
+            description("mandatory element missing")
+            display("mandatory element missing: ´{}´", t)
+        }
+    }
+}