assemblers: add org.osbuild.error

This is, like the stage with the same name, an assembler that
will exit with an error code (default 255, but can be specified
via the assembler options). It is mostly useful for testing.

assemblers/org.osbuild.error

@@ -0,0 +1,36 @@
+#!/usr/bin/python3
+"""
+Return an error
+
+Error assembler. Return the given error. Very much like the error stage this
+is useful for testing, debugging, and wasting time.
+"""
+
+
+import sys
+
+import osbuild.api
+
+
+SCHEMA = """
+"additionalProperties": false,
+"properties": {
+  "returncode": {
+    "description": "What to return code to use",
+    "type": "number",
+    "default": 255
+  }
+}
+"""
+
+
+def main(options):
+    errno = options.get("returncode", 255)
+    print(f"Error assembler will now return error: {errno}")
+    return errno
+
+
+if __name__ == '__main__':
+    args = osbuild.api.arguments()
+    r = main(args.get("options", {}))
+    sys.exit(r)