Return and propagate UECC_FAULT_DETECTED

This commit first changes the return convention of EccPoint_mult_safer() so
that it properly reports when faults are detected. Then all functions that
call it need to be changed to (1) follow the same return convention and (2)
properly propagate UECC_FAULT_DETECTED when it occurs.

Here's the reverse call graph from EccPoint_mult_safer() to the rest of the
library (where return values are translated to the MBEDTLS_ERR_ space) and test
functions (where expected return values are asserted explicitly).

EccPoint_mult_safer()
    EccPoint_compute_public_key()
        uECC_compute_public_key()
            pkparse.c
            tests/suites/test_suite_pkparse.function
        uECC_make_key_with_d()
        uECC_make_key()
            ssl_cli.c
            ssl_srv.c
            tests/suites/test_suite_pk.function
            tests/suites/test_suite_tinycrypt.function
    uECC_shared_secret()
        ssl_tls.c
        tests/suites/test_suite_tinycrypt.function
    uECC_sign_with_k()
        uECC_sign()
            pk.c
            tests/suites/test_suite_tinycrypt.function

Note: in uECC_sign_with_k() a test for uECC_vli_isZero(p) is suppressed
because it is redundant with a more thorough test (point validity) done at the
end of EccPoint_mult_safer(). This redundancy was introduced in a previous
commit but not noticed earlier.
diff --git a/library/pkparse.c b/library/pkparse.c
index 6a2507a..3f20225 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -987,7 +987,9 @@
     {
         ret = uECC_compute_public_key( keypair->private_key,
                                        keypair->public_key );
-        if( ret == 0 )
+        if( ret == UECC_FAULT_DETECTED )
+            return( MBEDTLS_ERR_PLATFORM_FAULT_DETECTED );
+        if( ret != UECC_SUCCESS )
             return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT );
     }