Add note to README about mrtest. Add check for running apk command. (MR 20)

README.md

@@ -2,6 +2,8 @@ This repository holds two programs, mrhlpr and mrtest. The mrhlpr program is
 described below, information about mrtest can be found in the
 [postmarketOS wiki](https://wiki.postmarketos.org/wiki/Mrtest).
 
+> The mrtest program must be run on pmOS or alpine device in order to work correctly.
+
 # mrhlpr - merge request helper
 
 Lightweight script to support maintainers of postmarketOS in the merge workflow on GitLab.

mrtest/apk_installed.py

@@ -20,7 +20,14 @@ def get_installed_all():
     ret = {}
     cmd = ["apk", "info", "-vv"]
     logging.debug(f"+ {cmd}")
-    output = subprocess.run(cmd, capture_output=True).stdout
+
+    try:
+        output = subprocess.run(cmd, capture_output=True).stdout
+    except FileNotFoundError:
+        print("ERROR: The command 'apk' was not found!\n"
+              "Please run the 'mrtest' on pmOS/alpine installation.")
+        exit(1)
+
     output = output.decode("utf-8")
     for line in output.split("\n"):
         if not line: