Improve diagnostics when non-String passed as a jstring.

Improve diagnostics when non-String passed as a jstring.

For my particular bug, the cause was obvious by inspection of the calling
code, but this might be useful in harder cases.

Before:
    W(17868) JNI WARNING: jstring 0x3600001 points to non-string object (Check_GetStringUTFChars)  (dalvikvm)

Change-Id: I883a99936b8c2cd79c5a691f7c335245ff50b34f
After:
    W(19680) JNI WARNING: jstring 0x3600001 points to object of type Llibcore/io/Posix; (Check_GetStringUTFChars)  (dalvikvm)