Resolve "Invalid signal name reports as "list index out of range""

Closes #19 (closed)

Problem

If either the signal name is invalid or attachment table record is missing, the pyCDB raises IndexError. This makes troubleshooting more complicated.

Analysis

In both case, the code assumes the corresponding records are present even though this is not enforced by any other means.

Solution

1. This fix adds custom RuntimeErrors pointing at the missing information.

Test

• This fix was tested while resolving webcdb-fastapi issue about adding continuous signal viewer (https://repo.tok.ipp.cas.cz/codac/webcdb-fastapi/-/issues/40).