build: add exit code for hash and check mismatches

Makes it easier to identify the failure reason in other tooling, eg.
differentiate between a non-deterministic --check vs a failed build.

	$ nix-build '<nix/fetchurl.nix>' --argstr url http://example.org --argstr sha256 0000000000000000000000000000000000000000000000000000
	hash mismatch in fixed-output derivation '/nix/store/nzi9ck45rwlxzcwr25is7qlf3hs5xl83-example.org':
	  wanted: sha256:0000000000000000000000000000000000000000000000000000
	  got:    sha256:08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm
	$ echo $?
	102

	$ nix-build -E 'with import <nixpkgs> {}; runCommand "foo" {} "date +%s > $out"' --check
	warning: rewriting hashes in '/nix/store/g3k47g0399fvjmbm0p0mnad74k4w8vkz-foo'; cross fingers
	error: derivation '/nix/store/mggc8dz13ackb49qca6m23zq4fpq132q-foo.drv' may not be deterministic: output '/nix/store/g3k47g0399fvjmbm0p0mnad74k4w8vkz-foo' differs
	$ echo $?
	104

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 0c5a73ea3..7a2af237e 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -855,10 +855,11 @@ CachedDownloadResult Downloader::downloadCached(
     }
 
     if (expectedStorePath != "" && storePath != expectedStorePath) {
+        unsigned int statusCode = 102;
         Hash gotHash = request.unpack
             ? hashPath(request.expectedHash.type, store->toRealPath(storePath)).first
             : hashFile(request.expectedHash.type, store->toRealPath(storePath));
-        throw nix::Error("hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
+        throw nix::Error(statusCode, "hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
             url, request.expectedHash.to_string(), gotHash.to_string());
     }