Improvise error reporting mechanism of cache_filters module

[jaladh-singhal]

In this PR, I've made load data functions to throw more accurate errors about the missing cache data. The several changes I made are:

• Taken out raise exception code from df_from_votable() into the functions that were using it: load_filter_index() & load_transmission_data() to customize error-checking as per need of caller function.
• In load_transmission_data(), absence of filter data file is not enough to decide filter_id doesn't index, but it should also be absent from index - for the cases when download of filter data left incomplete.
• Added a function download_transmission_data() to let users specifically download the missing filter data. Hence code to download transmission data in download_filter_data() is also wrapped in this new function.
• Also added more tests for:
  • the failing use cases by checking the expected error to be raised
  • the newly added function download_transmission_data() i.e. a part of complete downloading function: download_filter_data()

This is essential for the use of cache_filters module both independently and for general purpose, like in base.py