fix: return missing important information (#306)

Author: evilebottnawi

src/index.js

@@ -101,7 +101,7 @@ class UglifyJsPlugin {
  }
  }
 
- return warningMessage;
+ return `UglifyJs Plugin: ${warningMessage} in ${file}`;
  }
 
  apply(compiler) {

test/__snapshots__/all-options.test.js.snap

@@ -57,6 +57,6 @@ exports[`when applied with all options matches snapshot: manifest.d6857f782c13a9
 
 exports[`when applied with all options matches snapshot: warnings 1`] = `
 Array [
- "Dropping unused variable a [./test/fixtures/entry.js:4,0]",
+ "UglifyJs Plugin: Dropping unused variable a [./test/fixtures/entry.js:4,0] in main.0c220ec66316af2c1b24.js",
 ]
 `;