[bootstrap] show warning but not fail when install llvm@9 failed (#5751)

This commit allows bootstrap not to fail but print warning if
installing llvm@9 or clang-format-9 or clang-tidy-9 failed. This has
been the best practice for installing other pretty tools.

script/bootstrap

@@ -59,7 +59,7 @@ install_packages_apt()
     echo 'Installing pretty tools useful for code contributions...'
 
     # add clang-format and clang-tidy for pretty
-    sudo apt-get --no-install-recommends install -y clang-format-9 clang-tidy-9
+    sudo apt-get --no-install-recommends install -y clang-format-9 clang-tidy-9 || echo 'WARNING: could not install clang-format-9 and clang-tidy-9, which is useful if you plan to contribute C/C++ code to the OpenThread project.'
 
     # add yapf for pretty
     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.'
@@ -103,11 +103,11 @@ install_packages_brew()
     echo 'Installing pretty tools useful for code contributions...'
 
     # add clang-format for pretty
-    CLANG_FORMAT_VERSION="clang-format version 10"
+    CLANG_FORMAT_VERSION="clang-format version 9"
     command -v clang-format-9 || (command -v clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
         brew install llvm@9
         sudo ln -s "$(brew --prefix llvm@9)/bin/clang-format" /usr/local/bin/clang-format-9
-    }
+    } || echo 'WARNING: could not install llvm@9, which is useful if you plan to contribute C/C++ code to the OpenThread project.'
 
     # add yapf for pretty
     python3 -m pip install yapf || echo 'Failed to install python code formatter yapf. Install it manually if you need.'