Don't pass cover to ebook-convert if non existant

.gitignore

@@ -30,6 +30,6 @@ build/Release
 # Output of documentation
 docs/_book
 
-lib2
-test2
-theme2
+book.pdf
+book.epub
+book.mobi

lib/output/ebook.js

@@ -113,8 +113,9 @@ EbookOutput.prototype.locateCover = function() {
     cover = this.resolve(cover);
 
     // Cover doesn't exist and multilingual?
-    if (!fs.existsSync(cover) && this.parent) {
-        return this.parent.locateCover();
+    if (!fs.existsSync(cover)) {
+        if (this.parent) return this.parent.locateCover()
+        else return undefined;
     }
 
     return cover;

lib/utils/command.js

@@ -15,7 +15,7 @@ function exec(command, options) {
 
     var child = childProcess.exec(command, options, function(err, stdout, stderr) {
         if (!err) {
-            d.resolve();
+            return d.resolve();
         }
 
         err.message = stdout.toString('utf8') + stderr.toString('utf8');