Print top-level errors normally in `nix repl`

Previously, errors while printing values in `nix repl` would be printed
in `«error: ...»` brackets rather than displayed normally:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
«error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.»
```

Now, errors will be displayed normally if they're emitted at the
top-level of an expression:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
error:
       … in the condition of the assert statement
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/lib/customisation.nix:268:17:
          267|     in commonAttrs // {
          268|       drvPath = assert condition; drv.drvPath;
             |                 ^
          269|       outPath = assert condition; drv.outPath;

       … in the left operand of the OR (||) operator
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/passthrufun.nix:28:45:
           27|         if lib.isDerivation value then
           28|           lib.extendDerivation (valid value || throw "${name} should use `buildPythonPackage` or `toPythonModule` if it is to be part of the Python packages set.") {} value
             |                                             ^
           29|         else

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.
```

Errors emitted in nested structures (like e.g. when printing `nixpkgs`)
will still be printed in brackets.

Change-Id: I25aeddf08c017582718cb9772a677bf51b9fc2ad

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 4aa46dbc0..9811ea30c 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -167,7 +167,8 @@ struct NixRepl
             .force = true,
             .derivationPaths = true,
             .maxDepth = maxDepth,
-            .prettyIndent = 2
+            .prettyIndent = 2,
+            .errors = ErrorPrintBehavior::ThrowTopLevel,
         });
     }
 };