Rearrange parts of attr.rs that rustfmt has been refusing to format

[dtolnay]

Affected by this old rustfmt problem: rust-lang/rustfmt#3863

Currently on master, this is clearly not rustfmt-compatible formatting:

serde/serde_derive/src/internals/attr.rs

Lines 470 to 475 in 9af132f

	syn::Fields::Unnamed(_) | syn::Fields::Unit => {
	cx.error_spanned_by(
	fields,
	"#[serde(tag = \"...\")] can only be used on enums and structs with named fields",
	);
	}

Neither is this:

serde/serde_derive/src/internals/attr.rs

Lines 409 to 413 in 9af132f

	syn::Data::Enum(syn::DataEnum { enum_token, .. }) => cx
	.error_spanned_by(
	enum_token,
	"#[serde(default = \"...\")] can only be used on structs with named fields",
	),