refactor: improve hftest error message

Improve the error message in `hftest.py` if the JSON control string
cannot be found, or if the JSON cannot be parsed.

Change-Id: I03a5c6762f1fea34c168f5efc617ef0173ed2dd7
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/test/hftest/hftest.py b/test/hftest/hftest.py
index dac74e1..704c0fd 100755
--- a/test/hftest/hftest.py
+++ b/test/hftest/hftest.py
@@ -796,12 +796,22 @@
         test suites."""
         out = self.driver.run("json", "json", self.force_long_running)
         hf_out = self.extract_hftest_lines(out)
-        hf_out = hf_out[hf_out.index(HFTEST_CTRL_JSON_START) + 1
+        try:
+            hf_out = hf_out[hf_out.index(HFTEST_CTRL_JSON_START) + 1
                         :hf_out.index(HFTEST_CTRL_JSON_END)];
+        except ValueError as e:
+            print("Unable to find JSON control string:")
+            print(f"out={out}")
+            print(f"hf_out={hf_out}")
+            raise e
+
         hf_out = "\n".join(hf_out)
         try:
             return json.loads(hf_out)
         except ValueError as e:
+            print("Unable to parse JSON:")
+            print(f"out={out}")
+            print(f"hf_out={hf_outout}")
             print(out)
             raise e