fix: two crashes when building cutouts and using years outside the input data - #2283

Fixes #2156, which I reported in April.

Both issues surfaced when running the workflow with a locally built cutout for a historical weather year (1990). They are independent, so feel free to ask me to split this PR.

1. TypeError on the cutout path in rules/build_electricity.smk

dataset_version() in rules/common.smk builds the field with Path(...).as_posix(), so CUTOUT_DATASET["folder"] is a string. The build rule joins the filename with the / operator, which raises TypeError: unsupported operand type(s) for /: 'str' and 'str'. rules/retrieve.smk already uses + for the same variable, so this is an inconsistency between the two files. Only affects source: build, which is why it is easy to miss when using the pre-built cutouts.

2. KeyError instead of the intended fallback in attach_conventional_generators

The code selects the snapshot year from a conventional input file and falls back to the last column, with the comment "take last column if year selection fails". However df[year] raises KeyError for a year that is not among the columns, and KeyError is not caught, so the fallback never triggers for the most common failure case.

This is not limited to unusual years: data/nuclear_p_max_pu.csv covers 2009 to 2024 but skips 2014, so even a year inside the nominal range aborts the run.

Adding KeyError to the existing except clause restores the documented behaviour.