Implemented a more friendly stylus error handling

Added error-chain as build-dependency

Cargo.toml

@@ -40,6 +40,9 @@ ws = { version = "0.7", optional = true}
 [dev-dependencies]
 tempdir = "0.3.4"
 
+[build-dependencies]
+error-chain = "0.10"
+
 [features]
 default = ["output", "watch", "serve"]
 debug = []

build.rs

@@ -3,28 +3,81 @@
 use std::process::Command;
 use std::env;
 use std::path::Path;
+#[macro_use]
+extern crate error_chain;
 
-fn main() {
+error_chain!{
+ foreign_links {
+        Io(std::io::Error);
+    }
+}
+
+fn program_exists(program: &str) -> Result<()> {
+    Command::new(program)
+        .arg("-v")
+        .output()
+        .chain_err(|| format!("Please install '{}'!", program))?;
+    Ok(())
+}
+
+fn npm_package_exists(package: &str) -> Result<()> {
+    let status = Command::new("npm")
+        .args(&["list", "-g"])
+        .arg(package)
+        .output();
+
+    match status {
+        Ok(ref out) if out.status.success() => Ok(()),
+        _ => {
+            bail!("Missing npm package '{0}' \
+                  install with: 'npm -g install {0}'",
+                  package)
+        },
+    }
+}
+
+pub enum Resource<'a> {
+    Program(&'a str),
+    Package(&'a str),
+}
+use Resource::{Program, Package};
+
+impl<'a> Resource<'a> {
+    pub fn exists(&self) -> Result<()> {
+        match *self {
+            Program(name) => program_exists(name),
+            Package(name) => npm_package_exists(name),
+        }
+    }
+}
+
+fn run() -> Result<()> {
 
     if let Ok(_) = env::var("CARGO_FEATURE_REGENERATE_CSS") {
+        // Check dependencies
+        Program("npm").exists()?;
+        Program("node").exists()?;
+        Package("nib").exists()?;
+        Package("stylus").exists()?;
 
         // Compile stylus stylesheet to css
-        let manifest_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
-
+        let manifest_dir = env::var("CARGO_MANIFEST_DIR")
+            .chain_err(|| "Please run the script with: 'cargo build'!")?;
         let theme_dir = Path::new(&manifest_dir).join("src/theme/");
         let stylus_dir = theme_dir.join("stylus/book.styl");
 
         if !Command::new("stylus")
-                .arg(format!("{}", stylus_dir.to_str().unwrap()))
+                .arg(stylus_dir)
                 .arg("--out")
-                .arg(format!("{}", theme_dir.to_str().unwrap()))
+                .arg(theme_dir)
                 .arg("--use")
                 .arg("nib")
-                .status()
-                .unwrap()
+                .status()?
                 .success() {
-            panic!("Stylus encoutered an error");
+            bail!("Stylus encoutered an error");
         }
     }
+    Ok(())
+}
 
-}
+quick_main!(run);