fix(environment): raise FileNotFoundError instead of silently loading nothing

[boring877]

Problem

UnityPy.load("missing.unity3d") currently returns an empty Environment with no error. Typos and wrong paths hide behind env.objects == [] until much later in the script, where they surface as confusing "no objects found" behavior instead of the actual mistake.

There is even a commented-out raise FileNotFoundError for this exact case in load_file.

Why it is silent today

The silent skip is deliberate for internally resolved paths: #361 removed an over-eager FileNotFoundError that fired when dependency files were resolved from the current directory, and missing dependencies (packaging errors, references to Unity's global Library) are normal and must be skipped.

Change

This only affects top-level string paths passed explicitly to Environment / UnityPy.load: those are user intent, so resolving them to nothing now raises FileNotFoundError.

• Environment.__init__ raises when load_file returns None for a top-level path
• load_file itself is unchanged: dependency, split-file and case-insensitive fallbacks keep the silent-skip behavior Fixed loading assets from current directory #361 established
• relative paths still resolve against the previously set self.path before raising

Tests

• missing top-level file raises FileNotFoundError
• missing dependency stays silent
• folder loading and bytes loading are unaffected

Full test suite passes (52 tests).