fix: add informative messages to error classes

[java-james]

Summary

• NotInitializedError, FileNotFoundError, and EmptyEnvFileError now override toString() with actionable, context-rich messages instead of the unhelpful Instance of 'ClassName'
• FileNotFoundError and EmptyEnvFileError now carry the filename when available (e.g. FileNotFoundError: Environment file ".env" not found. Ensure the file exists and is listed under assets in pubspec.yaml.)
• loadFromString() without a filename correctly shows EmptyEnvFileError: The provided env string is empty.

Fixes or partially fixes #72, #127

Not a breaking change

• Class names and hierarchy (extends Error) are unchanged — existing on FileNotFoundError catch clauses are unaffected
• FileNotFoundError constructor uses an optional positional parameter ([this.filename]) — FileNotFoundError() still compiles
• EmptyEnvFileError constructor uses an optional named parameter ({this.filename}) — EmptyEnvFileError() still compiles

Test plan

• New test/errors_test.dart with 10 tests covering toString() output, field accessibility, integration with non-global DotEnv instances, and loadFromString() throw path
• All 53 tests pass (flutter test)
• Zero new analyzer warnings (flutter analyze)
• Example app unchanged — does not reference error classes directly