TJBench: Recover from non-fatal errors if possible

Previously, -stoponwarning only had an effect on the underlying
TurboJPEG C functions, but TJBench still aborted if a non-fatal error
occurred.  This commit modifies the C version of TJBench such that it
always recovers from a non-fatal error unless -stoponwarning is
specified.  Furthermore, the benchmark stores the details of the last
non-fatal error and does not print any subsequent non-fatal error
messages unless they differ from the last one.

Due to limitations in the Java API (specifically, the fact that it
cannot communicate errors, fatal or otherwise, to the calling program
without throwing a TJException), it was only possible to make
decompression operations fully recoverable within TJBench.  With other
operations, -stoponwarning still has an effect on the underlying C
library but has no effect at the Java level.

The Java API documentation has been amended to reflect that only certain
methods are truly recoverable, regardless of the state of
TJ.FLAG_STOPONWARNING.

java/org/libjpegturbo/turbojpeg/TJ.java

@@ -391,6 +391,11 @@ public final class TJ {
    * operation if the underlying codec throws a warning (non-fatal error).  The
    * default behavior is to allow the operation to complete unless a fatal
    * error is encountered.
+   * <p>
+   * NOTE: due to the design of the TurboJPEG Java API, only certain methods
+   * (specifically, {@link TJDecompressor TJDecompressor.decompress*()} methods
+   * with a void return type) will complete and leave the output image in a
+   * fully recoverable state after a non-fatal error occurs.
    */
   public static final int FLAG_STOPONWARNING = 8192;
   /**
@@ -409,6 +414,11 @@ public final class TJ {
   /**
    * The error was non-fatal and recoverable, but the image may still be
    * corrupt.
+   * <p>
+   * NOTE: due to the design of the TurboJPEG Java API, only certain methods
+   * (specifically, {@link TJDecompressor TJDecompressor.decompress*()} methods
+   * with a void return type) will complete and leave the output image in a
+   * fully recoverable state after a non-fatal error occurs.
    */
   public static final int ERR_WARNING = 0;
   /**