launcher: debian package to depend on curl | wget

also give informative error message when neither is available

launcher-package/build.sbt

@@ -191,7 +191,7 @@ val root = (project in file(".")).
     },
     // Used to have "openjdk-8-jdk" but that doesn't work on Ubuntu 14.04 https://github.com/sbt/sbt/issues/3105
     // before that we had java6-runtime-headless" and that was pulling in JDK9 on Ubuntu 16.04 https://github.com/sbt/sbt/issues/2931
-    debianPackageDependencies in Debian ++= Seq("bash (>= 3.2)"),
+    debianPackageDependencies in Debian ++= Seq("bash (>= 3.2)", "curl | wget"),
     debianPackageRecommends in Debian += "git",
     linuxPackageMappings in Debian += {
       val bd = sourceDirectory.value

sbt

@@ -123,6 +123,9 @@ download_url () {
       curl --silent -L "$url" --output "$jar"
     elif command -v wget > /dev/null; then
       wget --quiet -O "$jar" "$url"
+    else
+      echoerr "failed to download $url: Neither curl nor wget is avaialble"
+      exit 2
     fi
   } && [[ -f "$jar" ]]
 }