Remap source maps in JS DCE. Improve JS DCE error logging

See KT-19821

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -225,6 +225,9 @@ class Kotlin2JsGradlePluginIT : BaseGradleIT() {
         project.build("runRhino") {
             println(output)
             assertSuccessful()
+            val pathPrefix = "mainProject/build/min"
+            assertFileExists("$pathPrefix/exampleapp.js.map")
+            assertFileExists("$pathPrefix/examplelib.js.map")
         }
     }
 

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlin2JsDceProject/libraryProject/build.gradle

@@ -18,10 +18,12 @@ dependencies {
 }
 
 compileKotlin2Js.kotlinOptions.outputFile = "${buildDir}/examplelib.js"
+compileKotlin2Js.kotlinOptions.sourceMap = true
 
 jar {
     from buildDir
     include "**/*.js"
+    include "**/*.js.map"
 }
 
 jar.dependsOn(compileKotlin2Js)