Handle new assembly accession

[emiliorighi]

Decide how the CLI should behave if the user input an accession that is not present (yet) in the file with the aliases

[Max25R]

v1 handles three cases here:

• Sample names don't match anything in any assembly: auto-detection raises LowConfidenceDetection, user gets an error naming the top candidate and asking for explicit --from / --assembly.
• --assembly value not in DB: AssemblyNotFoundError, error message says "not found in the database. Check the --assembly value."
• Known assembly, but individual sequence names aren't in its alias rows: warn-and-pass-through (row kept unchanged in output, count + 5 examples printed at end).

Is this enough for what you had in mind?

[emiliorighi]

Good, create an extra check to make sure the provided assembly accession actually exists (HEAD or byte range GET on the ncbi ftp path: https://ftp.ncbi.nlm.nih.gov/genomes/all/GCA/963/855/885/: is 404/410?