Improve bin/detect error messages (#575)

bin/detect

 #!/usr/bin/env bash
 # bin/detect <build-dir>
 
+error() {
+  local c="2,999 s/^/ !     /"
+	# send all of our output to stderr
+	exec 1>&2
+
+	echo -e "\033[1;31m" # bold; red
+	echo -n " !     ERROR: "
+	# this will be fed from stdin
+  case $(uname) in
+		Darwin) sed -l "$c";; # mac/bsd sed: -l buffers on line boundaries
+		*)      sed -u "$c";; # unix/gnu sed: -u unbuffered (arbitrary) chunks of data
+	esac
+	echo -e "\033[0m" # reset style
+	exit 1
+}
+
 if [ -f $1/package.json ]; then
   echo 'Node.js'
   exit 0
 fi
 
->&2 echo 'Node.js: package.json not found in application root'
+if [[ -f "$1/.slugignore" ]] && grep -Fxq "package.json" "$1/.slugignore"; then
+  error << EOF
+'package.json' listed in '.slugignore' file
+
+The 'heroku/nodejs' buildpack is set on this application, but was
+unable to detect a 'package.json' file. This is likely because
+the '.slugignore' file is removing it before the build begins.
+
+For more information, refer to the following documentation:
+https://devcenter.heroku.com/articles/slug-compiler#ignoring-files-with-slugignore
+EOF
+elif [[ -f "$1/.gitignore" ]] && grep -Fxq "package.json" "$1/.gitignore"; then
+  error << EOF
+'package.json' listed in '.gitignore' file
+
+The 'heroku/nodejs' buildpack is set on this application, but was
+unable to detect a 'package.json' file. This is likely because
+the '.gitignore' file is preventing it from being checked in to
+the git repo.
+
+For more information, refer to the following documentation:
+https://devcenter.heroku.com/articles/gitignore
+EOF
+else
+  error <<- EOF
+Application not supported by 'heroku/nodejs' buildpack
+
+The 'heroku/nodejs' buildpack is set on this application, but was
+unable to detect a Node.js codebase.
+    
+A Node.js app on Heroku requires a 'package.json' at the root of
+the directory structure.
+
+If you are trying to deploy a Node.js application, ensure that this
+file is present at the top level directory. This directory has the
+following files:
+
+$(ls -1p $1)
+    
+If you are trying to deploy an application written in another
+language, you need to change the list of buildpacks set on your
+Heroku app using the 'heroku buildpacks' command.
+    
+For more information, refer to the following documentation:
+https://devcenter.heroku.com/articles/buildpacks
+https://devcenter.heroku.com/articles/nodejs-support#activation
+EOF
+fi
+
 exit 1

test/fixtures/gitignore-package-json/.gitignore

+package.json

test/fixtures/slugignore-package-json/.slugignore

+package.json

test/run

@@ -990,6 +990,22 @@ testMemoryMetrics() {
   assertFileNotContains "measure#buildpack.nodejs.exec.heroku-postbuild.memory=" $metrics_log
 }
 
+testBinDetectWarnings() {
+  detect "slugignore-package-json"
+  assertCapturedError "'package.json' listed in '.slugignore' file"
+  assertCapturedError "https://devcenter.heroku.com/articles/slug-compiler#ignoring-files-with-slugignore"
+
+  detect "gitignore-package-json"
+  assertCapturedError "'package.json' listed in '.gitignore' file"
+  assertCapturedError "https://devcenter.heroku.com/articles/gitignore"
+
+  detect "node-project-missing-package-json"
+  assertCapturedError "Application not supported by 'heroku/nodejs' buildpack"
+  assertCapturedError "https://devcenter.heroku.com/articles/nodejs-support#activation"
+  assertCapturedError "index.js"
+  assertCapturedError "src/"
+}
+
 # Utils
 
 pushd "$(dirname 0)" >/dev/null
@@ -1018,6 +1034,15 @@ default_process_types_cleanup() {
   fi
 }
 
+detect() {
+  default_process_types_cleanup
+  bp_dir=$(mktmpdir)
+  compile_dir=$(mktmpdir)
+  cp -a "$(pwd)"/* ${bp_dir}
+  cp -a ${bp_dir}/test/fixtures/$1/. ${compile_dir}
+  capture ${bp_dir}/bin/detect ${compile_dir}
+}
+
 compile() {
   default_process_types_cleanup
   bp_dir=$(mktmpdir)