Replace obvious additions of an `MBEDTLS_ERR_xxx` constant by a call to
`MBEDTLS_ERROR_ADD`.

Skip `case` statements since `MBEDTLS_ERROR_ADD(pp_constant)` is not a
preprocessor constant.

This commit does not replace additions split over lines. Those will be
handled in a subsequent commit.

```
git ls-files '*.h' '*.c' '*.function' '*.data' |
xargs perl -i -pe '
    next if /\bcase\b/;
    s/\b(MBEDTLS_ERR_\w+)\s*\+\s*(\w+)\b/MBEDTLS_ERROR_ADD($1, $2)/g;
    s/\b(\w+)\s*\+\s*(MBEDTLS_ERR_\w+)\b/MBEDTLS_ERROR_ADD($1, $2)/g'
```

Signed-off-by: Gilles Peskine <[email protected]>

Reject direct additions of error constants (regex-based approximation).

Fix the lone straggler.

Signed-off-by: Gilles Peskine <[email protected]>

Signed-off-by: Gilles Peskine <[email protected]>

What could possibly go wrong...

Signed-off-by: Gilles Peskine <[email protected]>

We mostly use MBEDTLS_ERROR_ADD with both a low-level and a high-level
error, but we also occasionally use it with a low-level code that can be 0.

Signed-off-by: Gilles Peskine <[email protected]>

Signed-off-by: Gilles Peskine <[email protected]>

Signed-off-by: Gilles Peskine <[email protected]>

Signed-off-by: Gilles Peskine <[email protected]>