[TAP] Fix handling of bad uploaded files:

- empty file
- not a valid VOTable document

In such cases, the following error message is returned:
  "The input file is not a valid VOTable document!"
A cause with more detais (especially the line and column numbers)
may be appended.

Cases handled with no error:

  - If the VOTable document has no rows, an empty table
    is uploaded. No error has to be returned.

  - If a row has a different number of columns than the number
    of declared FIELDs, additional values are ignored and missing
    values are replaced by NULL. This is actually nicely handled by
    STIL.