fix: raise ValueError on malformed YAML instead of leaking the parser's exception (#282)

[patchwright]

Problem

Fixes #282. When parse_yaml is given malformed YAML, the underlying parser's exception (PyYAML's yaml.scanner.ScannerError / ruamel's YAMLError) escapes as a raw stack trace instead of a clean error. Confirmed on master HEAD 5046af8.

Root cause

_pyyaml and _ruamel in yamale/readers/yaml_reader.py call yaml.load_all(...) without catching the parser's error, so it propagates straight out of parse_yaml.

Fix

Wrap each parser's load_all call in a try/except for its own error type (yaml.YAMLError for PyYAML, ruamel.yaml.YAMLError for ruamel) and re-raise as ValueError with the original message chained via from e. No API change; ~8 lines.

How to test

from yamale.readers import parse_yaml
parse_yaml(content='key: "unclosed', parser='pyyaml')
# before: yaml.scanner.ScannerError: while scanning a quoted scalar ... (stack trace)
# after:  ValueError: Could not parse YAML: while scanning a quoted scalar ...

Regression test added in yamale/readers/tests/test_bad_file.py, parametrized over pyyaml/PyYAML/ruamel. Full suite green (171 passed).

Backward compatibility

No breaking changes. Only the exception type for malformed input changes (raw parser error -> ValueError), which is the behavior requested in #282.

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Assisted-by: Claude (code generation, reviewed and tested locally)