Verify a valid HTTP client tool exists prior to installing Python

On Linux, Python installation requires that the Python code be downloaded.
In order to download Python, pyenv uses one of aria2c, curl, and wget.
When none of these is found, it fails to download Python.

This commit ensures that python_installer fast-fails so that
unnecessary activities such as cloning the pyenv repository need not
happen when a valid HTTP client tool is not found.

scripts/python_installer

@@ -64,6 +64,11 @@ function echo_with_indentation() {
     echo -e "   $message"
 }
 
+function echo_error_message () {
+    local message=$1
+    echo_with_indentation "\033[38;5;9m$message\033[0m"
+}
+
 function echo_recommendation_message() {
     local message=$1
     echo_with_indentation "\033[38;5;11m$message\033[0m"
@@ -146,6 +151,20 @@ function export_pyenv_and_pyenv_repository_paths() {
     export PATH="$PYENV_ROOT/bin:$PYENV_REPOSITORY_LOCATION/bin:$PATH"
 }
 
+function verify_http_client_exists() {
+    if type curl &>/dev/null; then
+        return
+    elif type aria2c &>/dev/null; then
+        return
+    elif [[ "$(wget --version 2>/dev/null || true)" = "GNU Wget 1.1"[0-3]* ]]; then
+        echo_error_message "wget (>= 1.14) or curl (>= 7.18.1) is required to download and install Python."
+        exit 1
+    else
+        echo_error_message "Could not find \`curl\`, \`wget\`, or \`aria2c\`. Please install one of these tools to continue Python installation"
+        exit 1
+    fi
+}
+
 function main() {
     echo_step_title "Determining whether pyenv is already installed and in PATH"
 
@@ -161,6 +180,9 @@ function main() {
     echo_step_title "Temporarily export necessary pyenv paths"
     export_pyenv_and_pyenv_repository_paths
 
+    echo_step_title "Checking whether Python can be downloaded (through curl, wget, or aria2c)"
+    verify_http_client_exists
+
     echo_step_title "Installing Python $PYTHON_VERSION"
     install_python