Allow a path prefix to be set for the JSON DB save/load location.

Improved the exception reported when constructing a JSON DB fails.
Merge from develop
 ChangeLog                   |  4 ----
 Cpp/fost-jsondb/blobdb.cpp  | 41 ++++++++---------------------------------
 Cpp/include/fost/jsondb.hpp |  6 +-----
 3 files changed, 9 insertions(+), 42 deletions(-)