Change spacing and font size of error text (#4415)

## Summary
<!-- Simple summary of what was changed. -->
Error text now 11pt (caption2)
8px after form instead of 4
## Motivation
<!-- Why are you making this change? If it's for fixing a bug, if
possible, please include a code snippet or example project that
demonstrates the issue. -->
https://jira.corp.stripe.com/browse/MOBILESDK-2884
## Testing
<!-- How was the code tested? Be as specific as possible. -->

![test_UpdatePaymentMethodViewControllerExpiredCard@3x](https://github.com/user-attachments/assets/e4acb6f7-6f79-480e-a7b2-f28d713eefcc)

## Changelog
<!-- Is this a notable change that affects users? If so, add a line to
`CHANGELOG.md` and prefix the line with one of the following:
    - [Added] for new features.
    - [Changed] for changes in existing functionality.
    - [Deprecated] for soon-to-be removed features.
    - [Removed] for now removed features.
    - [Fixed] for any bug fixes.
    - [Security] in case of vulnerabilities.
-->

StripeUICore/StripeUICore/Source/Elements/ElementsUI.swift

@@ -37,7 +37,7 @@ import UIKit
 
     public static func makeErrorLabel(theme: ElementsAppearance) -> UILabel {
         let label = UILabel()
-        label.font = theme.fonts.footnote
+        label.font = theme.fonts.error
         label.textColor = theme.colors.danger
         label.numberOfLines = 0
         label.setContentHuggingPriority(.required, for: .vertical)
@@ -100,6 +100,7 @@ import UIKit
                                             withTextStyle: .caption1,
                                             maximumPointSize: 20)
         public var footnote = UIFont.preferredFont(forTextStyle: .footnote, weight: .regular, maximumPointSize: 20)
+        public var error = UIFont.preferredFont(forTextStyle: .caption2, weight: .regular)
         public var smallFootnote = UIFont.preferredFont(forTextStyle: .caption2, weight: .medium)
         public var footnoteEmphasis = UIFont.preferredFont(forTextStyle: .footnote, weight: .medium, maximumPointSize: 20)
     }

StripeUICore/StripeUICore/Source/Elements/Section/SectionView.swift

@@ -37,6 +37,7 @@ final class SectionView: UIView {
         let stack = UIStackView(arrangedSubviews: [titleLabel, containerView, errorOrSubLabel])
         stack.axis = .vertical
         stack.spacing = 4
+        stack.setCustomSpacing(8, after: containerView)
         addAndPinSubview(stack)
 
         update(with: viewModel)