Improve error messages for YAML validation failures

[XDGFX]

Closes #505

Summary

When a WireViz YAML file contains a mistake, errors currently surface as opaque Python tracebacks with no indication of which connector, cable, or connection set caused the failure. This PR improves error reporting in three areas:

• wv_dataclasses.py: The style: simple pin count check now includes the connector's designator, the detected pin count, and a suggested fix rather than a bare message.
• wv_harness.py: Two new explicit guards — one for a pin label resolving to an out-of-bounds index (previously an unhandled IndexError), and one for a connection referencing an undefined cable (previously a KeyError).
• wireviz.py: Component generation and harness.connect() calls are wrapped to add context to any exception, identifying the connection set number and component names involved (e.g. "Error in connection set 3 (X1 → W1 → X2): ...").

These changes are purely additive to error handling and do not affect any output.

Test plan

• Run existing examples to confirm no regressions in output — all 16 examples pass
• Manually test a YAML with style: simple and multiple pins — error now reads: Connector 'X1' has style 'simple', which only supports one pin, but 3 pin(s) were specified. Either remove 'style: simple' or reduce to a single pin.
• Manually test a YAML with mismatched pinlabels/pins length — error now reads: X1: pin label 'C' (index 2) has no corresponding pin number — connector has 2 pin(s) but 3 pinlabel(s).
• Manually test a YAML referencing an undefined cable — error now reads: UNDEFINED_CABLE is an unknown template/designator/arrow. (caught at component generation, wrapped with connection set context)
• Manually test a YAML with a bad template in a connection set — connection set number and names appear in the error: Error in connection set 1 (X1 → NONEXISTENT → X1): ...