[JS IR BE] Add better error message when some function not found during init JsIrBackendContext

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsIrBackendContext.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -342,7 +342,7 @@ class JsIrBackendContext(
     private fun getIrClass(fqName: FqName): IrClassSymbol = symbolTable.referenceClass(getClass(fqName))
 
     internal fun getJsInternalFunction(name: String): SimpleFunctionDescriptor =
-        findFunctions(internalPackage.memberScope, Name.identifier(name)).single()
+        findFunctions(internalPackage.memberScope, Name.identifier(name)).singleOrNull() ?: error("Internal function '$name' not found")
 
     fun getFunctions(fqName: FqName): List<SimpleFunctionDescriptor> =
         findFunctions(module.getPackage(fqName.parent()).memberScope, fqName.shortName())