Add actionable error message for no npm on system during pip install

python/pminit/post-install-hook.py

@@ -1,5 +1,6 @@
 import subprocess
 import sys
+import shutil
 
 def execute(cmd: str):
     popen = subprocess.Popen(cmd, stdout = subprocess.PIPE, stderr = subprocess.STDOUT,
@@ -14,7 +15,25 @@ def execute(cmd: str):
         raise subprocess.CalledProcessError(return_code, cmd)
 
 def main():
-    execute("cd pythonmonkey && npm i --no-package-lock") # do not update package-lock.json
+    node_package_manager = 'npm'
+    # check if npm is installed on the system
+    if (shutil.which(node_package_manager) is None):
+        print("""
+
+PythonMonkey Build Error:
+
+
+  *    It appears npm is not installed on this system.
+  *    npm is required for PythonMonkey to build.
+  *    Please install NPM and Node.js before installing PythonMonkey.
+  *    Refer to the documentation for installing NPM and Node.js here: https://nodejs.org/en/download
+
+
+        """)
+        raise Exception("PythonMonkey build error: Unable to find npm on the system.")
+    else:
+        execute(f"cd pythonmonkey && {node_package_manager} i --no-package-lock") # do not update package-lock.json
 
 if __name__ == "__main__":
     main()
+