Checking random number generation for failures is pretty imporant [1, 2]
so let's make it easier for users to track such issues. Introduce a
SOTER_MUST_USE macro that adds an attribute to the function which will
cause a compiler warning if the return value is not used (or not
explicitly ignored by casting to void). See [3] for description.

Since we treat warnings as errors, CI compilation will fail like this:

    compile build/obj/tests/common/test_utils.c.o
    tests/common/test_utils.c:70:3: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                    soter_rand((uint8_t*)(&res), sizeof(size_t));
                    ^~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1 warning generated.

if anyone adds a soter_rand() call without checking the return value.

[1]: https://github.com/veorq/cryptocoding#use-strong-randomness
[2]: http://jbp.io/2014/01/16/openssl-rand-api
[3]: http://releases.llvm.org/9.0.0/tools/clang/docs/AttributeReference.html#nodiscard-warn-unused-result

We have this API, but it's not tested (or documented either). However,
it's there and we should at least exercise it in the test suite. Turns
out that it was not checking return value of soter_rand() and it's not
visible if there are no tests.

The API is... wacky, but that's what it is. I'm not really motivated to
clean it up at the moment (and that's out of scope).

Add missing error handling.

For ThemisPP throw an exception. For tests just abort with non-zero exit
code as the API does not allow for returning an error. That's actually
the case for ed25519 code too [1].

[1]: https://github.com/cossacklabs/themis/blob/5f886e1fa1742beb2070da93a2036ad79030dac5/src/soter/ed25519/gen_rand_32.c#L31-L35

Nothing interestring. I have found this typo when trying to run
clang-tidy manually. It is obscured during normal runs due to
"2>/dev/null" which is added there because clang-tidy likes to talk
even when no errors are detected and this is treated as warning output
by our build system.

Make sure that it goes first because when compiling for C++14 this gets
expanded into

    __attribute__((visibility("default")))
    [[nodiscard]]
    soter_status_t soter_rand(uint8_t* buffer, size_t length);

which throws Clang off-guard with this GCC compatibility syntax for
attributes. For some reason Clang thinks that [[nodiscard]] applies
only to "soter_status_t" which is not allowed.

Make sure that C++ attributes go before non-standard attributes.