TurboJPEG: Thread-safe error message retrieval

Introduce a new C API function (tjGetErrorStr2()) that can be used to
retrieve compression/decompression/transform error messages in a
thread-safe (i.e. instance-specific) manner.  Retrieving error messages
from global functions is still thread-unsafe.

Addresses a concern expressed in #151.

turbojpeg-mapfile.jni

@@ -90,3 +90,9 @@ TURBOJPEG_1.4
 		Java_org_libjpegturbo_turbojpeg_TJ_planeSizeYUV__IIIII;
 		Java_org_libjpegturbo_turbojpeg_TJ_planeWidth__III;
 } TURBOJPEG_1.3;
+
+TURBOJPEG_1.6
+{
+	global:
+		tjGetErrorStr2;
+} TURBOJPEG_1.4;