fix: reject a promise with an error object

- resolve "Unhandled promise rejection" error
- https://stackoverflow.com/a/47803880

lib/filter.js

@@ -120,8 +120,7 @@ function minifySvg () {
             if (options.logger) verbose.call(this, assetTxt, data, path, 'svg')
             resolve(route.set(path, data))
           } catch (err) {
-            reject(err)
-            throw new Error(err)
+            reject(new Error(err))
           }
         }
       })
@@ -156,8 +155,7 @@ function gzipFn () {
             if (options.logger) verbose.call(this, input, result, path, 'gzip')
             resolve(route.set(path + '.gz', result))
           } catch (err) {
-            reject(err)
-            throw new Error(err)
+            reject(new Error(err))
           }
         }
       })
@@ -191,8 +189,7 @@ function brotliFn () {
             if (options.logger) verbose.call(this, input, result, path, 'brotli')
             resolve(route.set(path + '.br', result))
           } catch (err) {
-            reject(err)
-            throw new Error(err)
+            reject(new Error(err))
           }
         }
       })