Clearer error message when HTTP download fails

Previously, if `http get` failed, the `download_tarball` function would
still continue since the ERR trap had no effect at that point.

Given a script in the form of `{ ... } || return 1`, the expressions
that are the part of the first group are not subject to ERR trap since
they are non-last in a chain of expressions.

However, since we still can't count on the ERR trap taking effect in
this phase, better just rewrite the function to manually abort.

Fixes #394

bin/ruby-build

@@ -283,9 +283,12 @@ download_tarball() {
 
   echo "-> $package_url" >&2
 
-  { http get "$package_url" "$package_filename"
-    verify_checksum "$package_filename" "$checksum"
-  } >&4 2>&1 || return 1
+  if http get "$package_url" "$package_filename" >&4 2>&1; then
+    verify_checksum "$package_filename" "$checksum" >&4 2>&1 || return 1
+  else
+    echo "error: failed to download $package_filename" >&2
+    return 1
+  fi
 
   if [ -n "$RUBY_BUILD_CACHE_PATH" ]; then
     local cached_package_filename="${RUBY_BUILD_CACHE_PATH}/$package_filename"

test/fetch.bats

+#!/usr/bin/env bats
+
+load test_helper
+export RUBY_BUILD_SKIP_MIRROR=1
+export RUBY_BUILD_CACHE_PATH=
+
+@test "failed download displays error message" {
+  stub curl false
+
+  install_fixture definitions/without-checksum
+  assert_failure
+  assert_output_contains "> http://example.com/packages/package-1.0.0.tar.gz"
+  assert_output_contains "error: failed to download package-1.0.0.tar.gz"
+}

test/test_helper.bash

@@ -94,3 +94,12 @@ assert_output() {
   fi
   assert_equal "$expected" "$output"
 }
+
+assert_output_contains() {
+  local expected="$1"
+  echo "$output" | grep -F "$expected" >/dev/null || {
+    { echo "expected output to contain $expected"
+      echo "actual: $output"
+    } | flunk
+  }
+}