Improve error propagation in stash

Stash was sometimes obscuring the actual error code, replacing it
with a -1 when there was more descriptive value.  This updates
stash to preserve the original error code more reliably along
with a variety of other error handling tweaks.

I believe this is an improvement, but arguably, preserving the
underlying error code may result in values that are harder to
interpret by the caller who does not understand the internals.
Discussion is welcome!

tests-clar/stash/drop.c

@@ -36,15 +36,22 @@ static void push_three_states(void)
 	cl_git_pass(git_repository_index(&index, repo));
 	cl_git_pass(git_index_add_from_workdir(index, "zero.txt"));
 	commit_staged_files(&oid, index, signature);
+	cl_assert(git_path_exists("stash/zero.txt"));
 
 	cl_git_mkfile("stash/one.txt", "content\n");
 	cl_git_pass(git_stash_save(&oid, repo, signature, "First", GIT_STASH_INCLUDE_UNTRACKED));
+	cl_assert(!git_path_exists("stash/one.txt"));
+	cl_assert(git_path_exists("stash/zero.txt"));
 
 	cl_git_mkfile("stash/two.txt", "content\n");
 	cl_git_pass(git_stash_save(&oid, repo, signature, "Second", GIT_STASH_INCLUDE_UNTRACKED));
+	cl_assert(!git_path_exists("stash/two.txt"));
+	cl_assert(git_path_exists("stash/zero.txt"));
 
 	cl_git_mkfile("stash/three.txt", "content\n");
 	cl_git_pass(git_stash_save(&oid, repo, signature, "Third", GIT_STASH_INCLUDE_UNTRACKED));
+	cl_assert(!git_path_exists("stash/three.txt"));
+	cl_assert(git_path_exists("stash/zero.txt"));
 
 	git_index_free(index);
 }