Also error if a user tries to insert invalid insertion positions

[Taepper]

#1039 resolved the issue that user's could ask for invalid insertion position and received an empty result instead of an error. While that has been fixed, the maintainer is still able to insert positions to SILO that have a position outside the range [0, n].

Note that although positions are 1-based, an insertion at 0 is allowed to insert before the first symbol. The semantics of "insertion at position x" is "insert after position x" as documented by nextstrain:

22030:ACT would indicate that the query sequence has the three bases ACT inserted between position 22030 and 22031 in the reference sequence (the indices are 1-based).

[fengelniederhammer]

Let's also explicitly clarify in the docs what the positions mean.