Small PK cleanups

- better error codes
- rm now-useless include
diff --git a/library/error.c b/library/error.c
index 333a8f8..0ea3c29 100644
--- a/library/error.c
+++ b/library/error.c
@@ -252,6 +252,8 @@
             snprintf( buf, buflen, "PK - Memory alloation failed" );
         if( use_ret == -(POLARSSL_ERR_PK_TYPE_MISMATCH) )
             snprintf( buf, buflen, "PK - Type mismatch, eg attempt to use a RSA key as EC, or to modify key type" );
+        if( use_ret == -(POLARSSL_ERR_PK_BAD_INPUT_DATA) )
+            snprintf( buf, buflen, "PK - Bad input parameters to function" );
 #endif /* POLARSSL_PK_C */
 
 #if defined(POLARSSL_PKCS12_C)