build.js: improve error handling

build.js

@@ -94,6 +94,11 @@ function build(opts,callback) {
     }
     var shell_args = ['rebuild'].concat(opts.args);
     var cmd = cp.spawn(shell_cmd,shell_args, {cwd: undefined, env: process.env, customFds: [ 0, 1, 2]});
+    cmd.on('error', function (err) {
+        if (err) {
+            return callback(new Error("Failed to execute '" + shell_cmd + ' ' + shell_args.join(' ') + "' (" + err + ")"));
+        }
+    });
     cmd.on('close', function (err, stdout, stderr) {
         if (err) {
             return callback(new Error("Failed to execute '" + shell_cmd + ' ' + shell_args.join(' ') + "' (" + err + ")"));
@@ -214,6 +219,7 @@ if (opts.force) {
                 util.download(from_shasum,{progress:false},function(err,expected_shasum_buffer) {
                     if (err) {
                         log(from_shasum + ' not found, skipping shasum check (' + err + ')');
+                        return done();
                     } else {
                         // now check shasum match
                         var expected = expected_shasum_buffer.toString().trim();
@@ -221,7 +227,6 @@ if (opts.force) {
                             return done(new Error("shasum does not match between remote and local: " + expected + ' ' + actual_shasum));
                         } else {
                             log('Sha1sum matches! ' + expected);
-                        }
                             // we are good: continue
                             log('Extracting to ' + opts.paths.runtime_folder);
                             new targz().extract(tmpfile, opts.paths.runtime_folder, function(err) {
@@ -235,6 +240,7 @@ if (opts.force) {
                                 }
                             });
                         }
+                    }
                 });
             });
         });