Print error messages but not traces

This makes output of values that include errors much cleaner.

Before:
```
nix-repl> { err = builtins.throw "uh oh!"; }
{ err = «error:
       … while calling the 'throw' builtin
         at «string»:1:9:
            1| { err = builtins.throw "uh oh!"; }
             |         ^

       error: uh oh!»; }
```

After:
```
nix-repl> { err = builtins.throw "uh oh!"; }
{ err = «error: uh oh!»; }
```

But if the whole expression throws an error, source locations and (if
applicable) a stack trace are printed, like you'd expect:

```
nix-repl> builtins.throw "uh oh!"
error:
       … while calling the 'throw' builtin
         at «string»:1:1:
            1| builtins.throw "uh oh!"
             | ^

       error: uh oh!
```

src/libexpr/print.cc

@@ -409,7 +409,7 @@ class Printer
     {
         if (options.ansiColors)
             output << ANSI_RED;
-        output << "«" << e.msg() << "»";
+        output << "«error: " << e.info().msg << "»";
         if (options.ansiColors)
             output << ANSI_NORMAL;
     }