Improved error handling

[ealmloff]

Specific Demand

• Reducing the number of unwraps

When optimizing assets, manganis currently unwraps in a few places that may fail (eg. opening a file here). We should return an error instead

• Moving away from anyhow

Anyhow is great for binaries and prototyping libraries, but it makes it difficult to handle different errors in CLIs using manganis. We should switch to an enum with explicit error types using thiserror

Implement Suggestion

• Bubble up more errors with ?
• Switch to an error enum with error derived using thiserror