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 <Gilles.Peskine@arm.com>
