Builder: real error messages instead of 'could not be read'

Every release includes all the fixes from the releases before it: always take the newest one. This one is small-vram with builder fixes only; the game itself is unchanged.

• "The game files could not be read" / "something went wrong": when the builder hit an error it didn't expect, it hid the reason. It now shows the actual error, and the full text is under Show details. If you still get stuck, please post that text in an issue so it can be fixed.
• A failed step is named again (it said "-u stopped", #154).
• A leftover code chunk from an earlier build no longer breaks the link with "duplicate symbol" (thanks to leonidasmark, PR #155, #153).
• Output with non-English letters in file names can no longer crash the builder.

Windows Defender / antivirus warnings: the builder is a new, unsigned program that starts other programs (Python, the compiler, the game), which is what Defender's generic "Trojan" verdict reacts to. It's a false positive; all of the source is in this repository. You can report it to Microsoft at https://www.microsoft.com/wdsi/filesubmission ("Incorrectly detected as malware").