expand_cmdsubst: Make more errors known

These printed "Unknown error while evaluating command substitution".

Now they print something like

```
fish: for: status: cannot overwrite read-only variable
for status in foo; end
    ^~~~~^
in command substitution
fish: Invalid arguments
echo (for status in foo; end)
     ^~~~~~~~~~~~~~~~~~~~~~~^
```

for `echo (for status in foo; end)`

This is, of course, still not *great*. Mostly the `fish: Invalid
arguments` is basically entirely redundant.

An alternative is to simply skip the error message, but that requires some
more scaffolding (describe_with_prefix adds some error messages on its
own, so we can't simply say "don't add the prefix if we don't have a
message")

(cherry picked from commit 1b5eec2af6)

src/expand.cpp

@@ -666,6 +666,23 @@ static expand_result_t expand_cmdsubst(wcstring input, const operation_context_t
             case STATUS_NOT_EXECUTABLE:
                 err = L"Command not executable";
                 break;
+            case STATUS_INVALID_ARGS:
+                // TODO: Also overused
+                // This is sent for:
+                // invalid redirections or pipes (like `<&foo`),
+                // invalid variables (invalid name or read-only) for for-loops,
+                // switch $foo if $foo expands to more than one argument
+                // time in a background job.
+                err = L"Invalid arguments";
+                break;
+            case STATUS_EXPAND_ERROR:
+                // Sent in `for $foo in ...` if $foo expands to more than one word
+                err = L"Expansion error";
+                break;
+            case STATUS_UNMATCHED_WILDCARD:
+                // Sent in `for $foo in ...` if $foo expands to more than one word
+                err = L"Unmatched wildcard";
+                break;
             default:
                 err = L"Unknown error while evaluating command substitution";
                 break;