stage2: Do not pop error trace if result is an error

This allows for errors to be "re-thrown" by yielding any error as the
result of a catch block. For example:

```zig
fn errorable() !void {
    return error.FallingOutOfPlane;
}

fn foo(have_parachute: bool) !void {
    return errorable() catch |err| b: {
        if (have_parachute) {
            // error trace will include the call to errorable()
            break :b error.NoParachute;
        } else {
            return;
        }
    };
}

pub fn main() !void {
    // Anything that returns a non-error does not pollute the error trace.
    try foo(true);

    // This error trace will still include errorable(), whose error was "re-thrown" by foo()
    try foo(false);
}
```

This is piece (2/3) of https://github.com/ziglang/zig/issues/1923#issuecomment-1218495574

test/stack_traces.zig

@@ -155,6 +155,59 @@ pub fn addCases(cases: *tests.StackTracesContext) void {
         },
     });
 
+    cases.addCase(.{
+        .name = "catch and re-throw error",
+        .source = 
+        \\fn foo() !void {
+        \\    return error.TheSkyIsFalling;
+        \\}
+        \\
+        \\pub fn main() !void {
+        \\    return foo() catch error.AndMyCarIsOutOfGas;
+        \\}
+        ,
+        .Debug = .{
+            .expect = 
+            \\error: AndMyCarIsOutOfGas
+            \\source.zig:2:5: [address] in foo (test)
+            \\    return error.TheSkyIsFalling;
+            \\    ^
+            \\source.zig:6:5: [address] in main (test)
+            \\    return foo() catch error.AndMyCarIsOutOfGas;
+            \\    ^
+            \\
+            ,
+        },
+        .ReleaseSafe = .{
+            .exclude_os = .{
+                .windows, // TODO
+                .linux, // defeated by aggressive inlining
+            },
+            .expect = 
+            \\error: AndMyCarIsOutOfGas
+            \\source.zig:2:5: [address] in [function]
+            \\    return error.TheSkyIsFalling;
+            \\    ^
+            \\source.zig:6:5: [address] in [function]
+            \\    return foo() catch error.AndMyCarIsOutOfGas;
+            \\    ^
+            \\
+            ,
+        },
+        .ReleaseFast = .{
+            .expect = 
+            \\error: AndMyCarIsOutOfGas
+            \\
+            ,
+        },
+        .ReleaseSmall = .{
+            .expect = 
+            \\error: AndMyCarIsOutOfGas
+            \\
+            ,
+        },
+    });
+
     cases.addCase(.{
         .name = "try return from within catch",
         .source =