Convert hard-coded messages into global constants.

[dataclouder]

There are more than 1,000 hard-coded strings in the library, many of which are repeated several times: error messages, type identifiers, debug headers, REST URL components.)
Using them as they are is error prone, and it is not facilitated by editors and IDEs, in addition to duplicating lots of literal values unnecessarily.

We should convert the immutable text into global constants, to be used consistently across the library.