Propagate missing `Cargo.toml` errors instead of unwrapping them

This allows them to be reported to the user, rather than panic and dump a
human-error failure.

Fixes #242

src/command/build.rs

@@ -64,12 +64,15 @@ pub struct BuildOptions {
     // build_config: Option<BuildConfig>,
 }
 
-impl From<BuildOptions> for Build {
+type BuildStep = fn(&mut Build, &Step, &Logger) -> Result<(), Error>;
-    fn from(build_opts: BuildOptions) -> Self {
+
+impl Build {
+    /// Construct a build command from the given options.
+    pub fn try_from_opts(build_opts: BuildOptions) -> Result<Self, Error> {
         let crate_path = set_crate_path(build_opts.path);
-        let crate_name = manifest::get_crate_name(&crate_path).unwrap();
+        let crate_name = manifest::get_crate_name(&crate_path)?;
         // let build_config = manifest::xxx(&crate_path).xxx();
-        Build {
+        Ok(Build {
             crate_path,
             scope: build_opts.scope,
             disable_dts: build_opts.disable_dts,
@@ -77,13 +80,9 @@ impl From<BuildOptions> for Build {
             debug: build_opts.debug,
             // build_config,
             crate_name,
+        })
     }
-    }
-}
-
-type BuildStep = fn(&mut Build, &Step, &Logger) -> Result<(), Error>;
 
-impl Build {
     /// Execute this `Build` command.
     pub fn run(&mut self, log: &Logger, mode: &BuildMode) -> Result<(), Error> {
         let process_steps = Build::get_process_steps(mode);

src/command/mod.rs

@@ -84,7 +84,7 @@ pub fn run_wasm_pack(command: Command, log: &Logger) -> result::Result<(), Error
                 "normal" => BuildMode::Normal,
                 _ => BuildMode::Normal,
             };
-            Build::from(build_opts).run(&log, &build_mode)
+            Build::try_from_opts(build_opts).and_then(|mut b| b.run(&log, &build_mode))
         }
         Command::Pack { path } => {
             info!(&log, "Running pack command...");