libutil: handle json builder log messages with unexpected format

Before this change, expressions like:

with import <nixpkgs> {};
runCommand "foo" {} ''
  echo '@nix {}' >&$NIX_LOG_FD
''

would result in Lix crashing, because accessing nonexistent fields of
a JSON object throws an exception.

Rather than handling each field individually, we just catch JSON
exceptions wholesale. Since these log messages are an unusual
circumstance, log a warning when this happens.

Fixes #544.

Change-Id: Idc2d8acf6e37046b3ec212f42e29269163dca893

tests/functional/dependencies.nix

@@ -31,6 +31,18 @@ let
     outputHashAlgo = "sha256";
     outputHash = "1dq9p0hnm1y75q2x40fws5887bq1r840hzdxak0a9djbwvx0b16d";
   };
+
+  unusual-logging = mkDerivation {
+    name = "unusual-logging";
+    buildCommand = ''
+      {
+        echo "@nix 1"
+        echo "@nix {}"
+        echo '@nix {"action": null}'
+      } >&$NIX_LOG_FD
+      touch $out
+    '';
+  };
 in
 mkDerivation {
   name = "dependencies-top";
@@ -40,6 +52,7 @@ mkDerivation {
   input1_drv = input1;
   input2_drv = input2;
   input0_drv = input0;
+  unusual_logging_drv = unusual-logging;
   fod_input_drv = fod_input;
   meta.description = "Random test package";
 }