Improving error/warning messages

[marcelortizv]

Implement the following to improve error handling:

• Zero overlap across subgroups. Send an informative error message to inform the user about this problem.
• Add OMEGA Conditioning Check (R/att_gt.R:262-265) to inform when Omega becomes computationally singular.
• Index Length Mismatch Warning (R/att_gt.R:239)