Add some functions to help with error reporting. They provided unified
error message format and convert Themis status code into human-readable
strings.

Secure Session and Secure Comparator code are handled separately
because some of their values overlap with generic codes.

Keys and messages must be non-empty. Themis does check for that but
it returns a generic 'invalid parameter' error. Perform early checks
and report more accurate errors.

Furthermore, check the argument count and their types and report
appropriate errors as well. Otherwise we risk crashing node.js process
due to assertion failure in node::Buffer::Data().

Rename "empty" variable to use distinct names across the tests because
this "this" thing in JavaScript is magic.

Co-Authored-By: ilammy <[email protected]>

Co-Authored-By: ilammy <[email protected]>

Use more descriptive error messages which suggestions on how to fix
the error. Drop the superfluous error code parameter which is always
the same. We're going to use these short forms only for parameter
validation anyway.

We're going to improve the exception error messages so remove all these
hardcoded "invalid parameter" strings for now. We'll replace that with
some error verification callback later.

If we have a status code, add it as a "code" property to the Error
object we throw. Verify the code in tests where appropriate.

Add THEMIS_INVALID_PARAMETER status code to all early parameter checks
that we perform. Now all Themis errors will have a "code" property.
Rename the helper function accordinly.

Recently released mocha 6.0.0 is not compatible with Node in our CI
environment. Pin the version to ^5.2.0 while it is still available.

Call() method on callbacks has been deprecated, it is suggested to use
Nan::Call() function for synchronous calls. Do this to avoid warnings
about using deprecated API, who knows when it's going to be removed.

It turned out that public key callback did not have enough checks and
could crash Node process if the user returns unexpected value from it.
Add type checks for result: an array is okay, null and undefined are
explicit "not found" values, anything else is also considered "not
found" but with a different error code (the exact value does not matter
at the moment). Also add a length check to avoid buffer overflow when
doing memcpy().

Commit 6caa83c contains local changes
which were not intended to be committed. Revert them.