[PTFM-6116] [PTFM-6308] [PTFM-6318] Various dxcpp and UA changes

  - Better error handling (more classes, etc) in dxcpp
  - Better default values in UA
  - Better handling of bad_alloc and schannel error in UA
  - Updated logging class in dxcpp
  - Various other minor changes
  - UA version bumped to 1.2.6
Merge pull request #20 from dnanexus/PTFM-6117-6123-6116-dxcpp-changes

[PTFM-6116] [PTFM-6308] [PTFM-6318] Various dxcpp and UA changes