[bootstrap] update bootstrap user messages (#5161)

- Indicate that pretty tools are primarily used for code contributions.

- Change to bash.

script/bootstrap

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 #  Copyright (c) 2017, The OpenThread Authors.
 #  All rights reserved.
@@ -31,14 +31,18 @@
 #      example compilation and programming.
 #
 
-set -e
+set -euxo pipefail
 
 install_packages_apt()
 {
+    echo 'Installing toolchain dependencies...'
+
     # apt-get update and install dependencies
     sudo apt-get update
     sudo apt-get --no-install-recommends install -y automake g++ libtool lsb-release make cmake ninja-build shellcheck
 
+    echo 'Installing GNU Arm Embedded Toolchain...'
+
     PLATFORM=$(lsb_release -is)
     RELEASE=$(lsb_release -rs)
     UBUNTU2004=20.04
@@ -56,17 +60,19 @@ install_packages_apt()
         sudo apt-get --no-install-recommends install -y gcc-arm-embedded
     fi
 
+    echo 'Installing pretty tools useful for code contributions...'
+
     # add clang-format for pretty
     sudo apt-get --no-install-recommends install -y clang-format-6.0
 
     # add yapf for pretty
-    python3 -m pip install yapf==0.29.0 || echo 'Failed to install python code formatter yapf. Install it manually if you need.'
+    python3 -m pip install yapf==0.29.0 || echo 'WARNING: could not install yapf, which is useful if you plan to contribute python code to the OpenThread project.'
 
     # add mdv for local size report
-    python3 -m pip install mdv || echo 'Failed to install markdown render. Install it manually if you need.'
+    python3 -m pip install mdv || echo 'WARNING: could not install mdv, which is useful if you plan to contribute markdown to the OpenThread project.'
 
     # add shfmt for shell pretty, try brew only because snap does not support home directory not being /home and doesn't work in docker.
-    command -v shfmt || brew install shfmt || echo 'Failed to install shfmt. Install it manually if you need.'
+    command -v shfmt || brew install shfmt || echo 'WARNING: could not install shfmt, which is useful if you plan to contribute shell scripts to the OpenThread project.'
 }
 
 install_packages_opkg()
@@ -81,9 +87,13 @@ install_packages_rpm()
 
 install_packages_brew()
 {
+    echo 'Installing toolchain dependencies...'
+
     # add build tools
     brew install automake libtool cmake ninja shfmt shellcheck
 
+    echo 'Installing GNU Arm Embedded Toolchain...'
+
     # add ARM toolchain
     brew tap ArmMbed/homebrew-formulae
     brew install arm-none-eabi-gcc
@@ -94,6 +104,8 @@ install_packages_brew()
         echo 'warning: please install Command Line Tools from https://developer.apple.com/download/more/'
     fi
 
+    echo 'Installing pretty tools useful for code contributions...'
+
     # add clang-format for pretty
     CLANG_FORMAT_VERSION="clang-format version 6.0"
     command -v clang-format-6.0 || (command -v clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
@@ -128,6 +140,7 @@ install_packages()
 main()
 {
     install_packages
+    echo 'bootstrap completed successfully.'
 }
 
 main