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/tests/suites/test_suite_pkparse.function b/tests/suites/test_suite_pkparse.function
index 5563d60..54fa7af 100644
--- a/tests/suites/test_suite_pkparse.function
+++ b/tests/suites/test_suite_pkparse.function
@@ -137,7 +137,7 @@
         uecckey = mbedtls_pk_uecc( ctx );
         TEST_ASSERT( uECC_valid_public_key( uecckey->public_key ) == 0 );
         TEST_ASSERT( uECC_compute_public_key( uecckey->private_key,
-                                              tmp_pubkey ) != 0 );
+                                              tmp_pubkey ) == UECC_SUCCESS );
         TEST_ASSERT( memcmp( tmp_pubkey, uecckey->public_key,
                             sizeof( tmp_pubkey ) ) == 0 );
 #endif /* MBEDTLS_USE_TINYCRYPT */