Raise Unit::ParseError instead of built-in SyntaxError

[nertzy]

Summary

• Malformed unit expressions (unbalanced parentheses, dangling operators) previously raised Ruby's built-in SyntaxError. That is the wrong class for a bad argument value:
  • SyntaxError is a ScriptError, not a StandardError, so it slips through a plain rescue and is awkward to handle.
  • It conflates a malformed input string with a Ruby-level source parse failure.
• Introduce Unit::ParseError < ArgumentError and raise it from the expression parser instead of SyntaxError. A malformed unit string is a bad argument, so ArgumentError is the correct lineage:
  • it's a StandardError (caught by a bare rescue), and
  • it's distinct from Unit::IncompatibleUnitError (a TypeError), keeping value errors and type errors separate.
• Updated the parse_unit RDoc to document the new exception class.

Test plan

• bundle exec rake (runs the suite twice — spec:no_dsl and spec:dsl) — green on both load paths
• New spec/error_spec.rb examples assert Unit::ParseError is an ArgumentError/StandardError, is caught by a plain rescue, and is not a TypeError