Show user guidance to configuration file if file not exists

src/main/scala/com/github/tanishiking/scalaunfmt/cli/Cli.scala

@@ -1,6 +1,7 @@
 package com.github.tanishiking.scalaunfmt.cli
 
 import java.io.PrintStream
+import java.nio.file.NoSuchFileException
 
 import com.github.tanishiking.scalaunfmt.core.Runner
 import metaconfig.{Conf, Configured}
@@ -22,7 +23,17 @@ object Cli {
 
     val runner = new Runner(errStream)
 
-    Conf.parseFile(confPath.toFile) match {
+    val conf = try {
+      Conf.parseFile(confPath.toFile)
+    } catch {
+      case e: NoSuchFileException =>
+        throw new IllegalArgumentException(
+          s"""Configuration file ${e.getFile} not found.
+            |Provide the file in reference to https://github.com/tanishiking/scalaunfmt#configuration""".stripMargin
+        )
+    }
+
+    conf match {
       case Configured.Ok(value) =>
         runner.run(
           files,

src/test/scala/com/github/tanishiking/scalaunfmt/cli/CliSpec.scala

@@ -1,13 +1,25 @@
 package com.github.tanishiking.scalaunfmt.cli
 
 import java.io.{ByteArrayOutputStream, PrintStream}
-import java.nio.file.Files
+import java.nio.file.{Files, NoSuchFileException, Paths}
 
 import org.scalatest.{FunSpec, Matchers}
 
 class CliSpec extends FunSpec with Matchers {
   describe("Cli") {
     describe("run") {
+      it("should show error message to guide users to create a config file") {
+        val configPath = Paths.get("NO.SUCH.FILE.conf")
+        val opt = CliOptions(
+          config = configPath,
+          version = "2.0.0-RC5"
+        )
+        val ex = intercept[IllegalArgumentException] {
+          Cli.run(opt, System.out, System.err)
+        }
+        assert(ex.getMessage.contains("Configuration file NO.SUCH.FILE.conf not found."))
+      }
+
       it("should throw exception for unparsable config") {
         val configPath = Files.createTempFile("temp", ".conf")
         val unparsableConf =