JSON-RPC error messages

[nicreuss]

These changes try to align the error messages generated in the mjson jsonrpc handler with the error messages in the JSON-RPC 2.0 Specification. An example of such behavior is seen in #60.

Additionally, while writing tests for this new functionality using the existing test pattern, I observed a situation where some tests for the jsonrpc functionality would pass even when they contained an intentional error. When there were two consecutive tests that produced the same output string, the second test would not fail (when it should have) because the call to ASSERT would compare strings using the output string from the previous unit test without first checking the length. Since the length is set to zero before each test, it should be tested using ASSERT before performing a string comparison.