BUG: validate to_offset aliases before stride parsing

[sun-0-set]

• follows up on ENH: improve to_offset error message for deprecated uppercase aliases #65428
• Tests added and passed if fixing a bug or adding a new feature
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When parsing fractional strides with an invalid non-tick alias, to_offset attempts to parse the stride as an integer before validating the alias, exposing a lower-level invalid literal for int() error for inputs like "2.5foo", which obfuscates the actual error (invalid alias).

This runs offset=_get_offset() early to validate non-tick offset names before going into stride parsing.

Note: This is not elegant in the least, but imo does the job. A broader fix would entail rethinking the validation and error reporting logic for invalid and deprecated aliases/suffixes.

Validation: python -m pytest pandas/tests/tslibs/test_to_offset.py -q

[jbrockmendel]

cc @natmokval