[NFC] Provide exception overloads without context

[falbrechtskirchinger]

Currently the static create() member functions of exceptions require a const BasicJsonType &context parameter. This PR adds overloads without a context parameter and replaces uses of JSON_THROW(exception_type::create(..., BasicJsonType())) and similar with the context-less version.

This removes one obstacle to freeing json_pointer from its class template parameter in the future.

[coveralls]

Coverage decreased (-0.05%) to 99.946% when pulling 698ccc4 on falbrechtskirchinger:exceptions-no-context into e4643d1 on nlohmann:develop.

[gregmarr]

I'm not wild about the duplication, and this makes it easier to forget to see if there is an appropriate context parameter available. What about changing it from a reference to a pointer so that the callers that don't have one can just pass nullptr?

[falbrechtskirchinger]

Fair enough. I don't like the duplication either. This is a low-priority item and I'll get to it eventually.

[falbrechtskirchinger]

This has been folded into #3415 with changes requested.