return Error from minify() (#1880)

Have `minify()` return `Error` in `result.error` rather than throwing it.

lib/minify.js

@@ -146,6 +146,8 @@ function minify(files, options) {
             result.warnings = warnings;
         }
         return result;
+    } catch (ex) {
+        return { error: ex };
     } finally {
         AST_Node.warn_function = warn_function;
     }