Add traces to errors while updating flake lock file

Example:

$ nix build --show-trace
error: unable to download 'https://api.github.com/repos/NixOS/nixpkgs/commits/no-such-branch': HTTP error 422 ('')

       response body:

       {
         "message": "No commit found for SHA: no-such-branch",
         "documentation_url": "https://docs.github.com/rest/reference/repos#get-a-commit"
       }

       … while fetching the input 'github:NixOS/nixpkgs/no-such-branch'

       … while updating the flake input 'nixpkgs'

       … while updating the lock file of flake 'git+file:///home/eelco/Dev/nix'

src/libexpr/flake/flake.cc

@@ -298,6 +298,8 @@ LockedFlake lockFlake(
 
     auto flake = getFlake(state, topRef, lockFlags.useRegistries, flakeCache);
 
+    try {
+
         // FIXME: symlink attack
         auto oldLockFile = LockFile::read(
             flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir + "/flake.lock");
@@ -351,6 +353,8 @@ LockedFlake lockFlake(
                 auto inputPathS = printInputPath(inputPath);
                 debug("computing input '%s'", inputPathS);
 
+                try {
+
                     /* Do we have an override for this input from one of the
                        ancestors? */
                     auto i = overrides.find(inputPath);
@@ -490,6 +494,11 @@ LockedFlake lockFlake(
                                 std::make_shared<LockedNode>(lockedRef, *input.ref, false));
                         }
                     }
+
+                } catch (Error & e) {
+                    e.addTrace({}, "while updating the flake input '%s'", inputPathS);
+                    throw;
+                }
             }
         };
 
@@ -576,6 +585,11 @@ LockedFlake lockFlake(
         }
 
         return LockedFlake { .flake = std::move(flake), .lockFile = std::move(newLockFile) };
+
+    } catch (Error & e) {
+        e.addTrace({}, "while updating the lock file of flake '%s'", flake.lockedRef.to_string());
+        throw;
+    }
 }
 
 void callFlake(EvalState & state,

src/libfetchers/fetchers.cc

@@ -132,7 +132,14 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
         }
     }
 
-    auto [tree, input] = scheme->fetch(store, *this);
+    auto [tree, input] = [&]() -> std::pair<Tree, Input> {
+        try {
+            return scheme->fetch(store, *this);
+        } catch (Error & e) {
+            e.addTrace({}, "while fetching the input '%s'", to_string());
+            throw;
+        }
+    }();
 
     if (tree.actualPath == "")
         tree.actualPath = store->toRealPath(tree.storePath);