🐛 Print user messages to stderr (#27)

Previously, any messages only intended for the user (status messages,
warnings, errors) were sent over stdout. The actual emoji items are also
sent to the pickers over stdout so they would sometimes pick up the
messages and show them as an option to pick. If history is enabled, we
then also save the falsely included items there to forever display as an
option.

This commit changes internal messages to all be sent over stderr
instead, so they should not be picked up by any external programs we
hand off to.

test/bemoji_errors.bats

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+
+setup_file() {
+    # make bemoji executable from anywhere relative to current testfile
+    TEST_DIR="$( cd "$( dirname "$BATS_TEST_FILENAME" )" >/dev/null 2>&1 && pwd )"
+    PATH="$TEST_DIR/..:$PATH"
+}
+
+setup() {
+    load 'test_helper/bats-support/load'
+    load 'test_helper/bats-assert/load'
+
+    # set up small default set of test emoji for each test
+    export BEMOJI_DB_LOCATION="$BATS_TEST_TMPDIR/database"
+    export BEMOJI_HISTORY_LOCATION="$BATS_TEST_TMPDIR/history"
+    mkdir -p "$BEMOJI_DB_LOCATION" "$BEMOJI_HISTORY_LOCATION"
+    cat "$BATS_TEST_DIRNAME/resources/test_emoji.txt" > "$BEMOJI_DB_LOCATION/emoji.txt"
+
+    # these tests require stdout to be separated from stderr
+    # such run flags were only introduced in recent bats version
+    bats_require_minimum_version 1.5.0
+}
+
+@test "Prints clipper error to stderr" {
+    BEMOJI_PICKER_CMD="echo hi" run --separate-stderr bemoji 3>&-
+    assert_output ""
+    output="$stderr"
+    assert_output "No suitable clipboard tool found."
+}
+
+@test "Prints picker error to stderr" {
+    run --separate-stderr bemoji 3>&-
+    assert_output ""
+    output="$stderr"
+    assert_output "No suitable picker tool found."
+}
+
+@test "Prints typer error to stderr" {
+    BEMOJI_PICKER_CMD="echo hi" run --separate-stderr bemoji -t 3>&-
+    assert_output ""
+    output="$stderr"
+    assert_output "No suitable typing tool found."
+}