LibCore: Propagate errors from Stream::*_entire_buffer

Author: https://github.com/timschumi
Commit: 9a3e95785e
Pull-request: https://github.com/SerenityOS/serenity/pull/16429
Reviewed-by: https://github.com/sin-ack ✅

Tests/LibCpp/test-cpp-preprocessor.cpp

@@ -17,8 +17,7 @@ static DeprecatedString read_all(DeprecatedString const& path)
     auto file = MUST(Core::Stream::File::open(path, Core::Stream::OpenMode::Read));
     auto file_size = MUST(file->size());
     auto content = MUST(ByteBuffer::create_uninitialized(file_size));
-    if (!file->read_entire_buffer(content.bytes()))
-        VERIFY_NOT_REACHED();
+    MUST(file->read_entire_buffer(content.bytes()));
     return DeprecatedString { content.bytes() };
 }