lib/asserts: use throw to display message for assertMsg

`assert` has the annoying property that it dumps a lot of code at the
user without the built in capability to display a nicer message. We have
worked around this using `assertMsg` which would *additionally* display
a nice message. We can do even better: By using `throw` we can make
evaluation fail before assert draws its conclusions and prevent it from
displaying the code making up the assert condition, so we get the nicer
message of `throw` and the syntactical convenience of `assert`.

Before:

    nix-repl> python.override { reproducibleBuild = true; stripBytecode = false; }
    trace: Deterministic builds require stripping bytecode.
    error: assertion (((lib).assertMsg  (reproducibleBuild -> stripBytecode))  "Deterministic builds require stripping bytecode.") failed at /home/lukas/src/nix/nixpkgs/pkgs/development/interpreters/python/cpython/2.7/default.nix:45:1

After:

    nix-repl> python.override { reproducibleBuild = true; stripBytecode = false; }
    error: Deterministic builds require stripping bytecode.

lib/asserts.nix

@@ -2,35 +2,33 @@
 
 rec {
 
-  /* Print a trace message if pred is false.
+  /* Throw if pred is false, else return pred.
      Intended to be used to augment asserts with helpful error messages.
 
      Example:
        assertMsg false "nope"
-       => false
-       stderr> trace: nope
+       stderr> error: nope
 
-       assert (assertMsg ("foo" == "bar") "foo is not bar, silly"); ""
-       stderr> trace: foo is not bar, silly
-       stderr> assert failed at …
+       assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
+       stderr> error: foo is not bar, silly
 
      Type:
        assertMsg :: Bool -> String -> Bool
   */
   # TODO(Profpatsch): add tests that check stderr
   assertMsg = pred: msg:
-    if pred
-    then true
-    else builtins.trace msg false;
+    pred || builtins.throw msg;
 
   /* Specialized `assertMsg` for checking if val is one of the elements
      of a list. Useful for checking enums.
 
      Example:
-       let sslLibrary = "libressl"
+       let sslLibrary = "libressl";
        in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
-       => false
-       stderr> trace: sslLibrary must be one of "openssl", "bearssl", but is: "libressl"
+       stderr> error: sslLibrary must be one of [
+       stderr>   "openssl"
+       stderr>   "bearssl"
+       stderr> ], but is: "libressl"
 
      Type:
        assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool