Properly report `swift package describe` stdout & stderr

[stackotter]

When swift package describe --type json fails, Swift Bundler attempts to report its stdout and stderr. However, it prints out nothing, and in reality swift package describe --type json prints out proper error informations, as seen below.

$ sbun run
info: Loading package manifest
error: Process didn't exit successfully: `/usr/bin/env swift package describe --type json` (exit status: 1)
--- stdout & stderr

$ swift package describe --type json
error: 'stackwithunconstrainedcrossaxis': Invalid manifest (compiled with: ["/Volumes/stackotter/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc", "-vfsoverlay", "/var/folders/rg/rbnmrj9s25906hk5f1v8zv8r0000gn/T/TemporaryDirectory.UMDdFN/vfs.yaml", "-L", "/Volumes/stackotter/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/pm/ManifestAPI", "-lPackageDescription", "-Xlinker", "-rpath", "-Xlinker", "/Volumes/stackotter/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/pm/ManifestAPI", "-target", "arm64-apple-macosx14.0", "-sdk", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk", "-F", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks", "-F", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/PrivateFrameworks", "-I", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib", "-L", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib", "-swift-version", "5", "-I", "/Volumes/stackotter/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/pm/ManifestAPI", "-sdk", "/Volumes/stackotter/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk", "-package-description-version", "5.5.0", "/Users/stackotter/Desktop/Projects/SwiftCrossUI/EdgeCases/StackWithUnconstrainedCrossAxis/Package.swift", "-o", "/var/folders/rg/rbnmrj9s25906hk5f1v8zv8r0000gn/T/TemporaryDirectory.ZI5GPJ/stackwithunconstrainedcrossaxis-manifest"])
/Users/stackotter/Desktop/Projects/SwiftCrossUI/EdgeCases/StackWithUnconstrainedCrossAxis/Package.swift:7:25: error: 'v13' is unavailable
 5 | let package = Package(
 6 |     name: "StackWithUnconstrainedCrossAxis",
 7 |     platforms: [.macOS(.v13), .iOS(.v14), .tvOS(.v14)],
   |                         `- error: 'v13' is unavailable
 8 |     dependencies: [],
 9 |     targets: [

PackageDescription.SupportedPlatform.MacOSVersion.v13:4:23: note: 'v13' was introduced in PackageDescription 5.7
2 |   struct MacOSVersion {
3 | @available(_PackageDescription 5.7)
4 |     public static let v13: SupportedPlatform.MacOSVersion  }
  |                       `- note: 'v13' was introduced in PackageDescription 5.7
5 | }
6 |
error: fatalError