improve api tests and error output

show only relevant aptly logs if a test fails.
for async tasks, show task output, as it contains the error message.

system/t12_api/gpg.py

@@ -7,11 +7,16 @@ from api_lib import APITest
 
 
 def check_gpgkey_exists(gpg_key, keyring):
-    subprocess.check_call([
+    p = subprocess.Popen([
         "gpg", "--no-default-keyring",
         "--keyring", keyring,
-        "--fingerprint", gpg_key,
-    ])
+        "--fingerprint", gpg_key],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+    p.communicate()
+    if p.returncode != 0:
+        raise Exception("gpg key does not exists")
 
 
 class GPGAPITestAddKey(APITest):