add error trace helpers

[sreimers]

Usage example:

#include <re.h>

int fn1(void);
int fn2(void);
int fn3(void);

int fn1(void) {
	int err;

	err = fn2();
	IF_ERR_RETURN(err);

	return 0;
}

int fn2(void) {
	int err;

	err = fn3();
	IF_ERR_RETURN(err);

	return 0;
}

int fn3(void) {
	int err;

	err = EINVAL;
	IF_ERR_RETURN(err);

	return 0;
}

int main(void)
{
	int err;

	err = fn1();
	IF_ERR_GOTO_OUT(err);

	return 0;
out:
	re_printf("error %d\n", err);
        return err;
}

make TRACE_ERR=1

Output after execution:

TRACE_ERR: src/main.c:29: fn3(): Invalid argument (22)
TRACE_ERR: src/main.c:20: fn2(): Invalid argument (22)
TRACE_ERR: src/main.c:11: fn1(): Invalid argument (22)
TRACE_ERR: src/main.c:39: main(): Invalid argument (22)
error 22

[codecov]

Codecov Report

Merging #87 (1a7379a) into master (00fdc24) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 1a7379a differs from pull request most recent head 221965f. Consider uploading reports for the commit 221965f to get more accurate results

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   53.30%   53.31%           
=======================================
  Files         192      192           
  Lines       21473    21474    +1     
=======================================
+ Hits        11447    11449    +2     
+ Misses      10026    10025    -1     

Impacted Files	Coverage Δ
src/dns/dname.c	72.04% <0.00%> (-3.23%)	⬇️
src/list/list.c	85.29% <0.00%> (-0.74%)	⬇️
src/mbuf/mbuf.c	79.06% <0.00%> (+0.12%)	⬆️
src/sip/transp.c	37.39% <0.00%> (+0.43%)	⬆️
src/json/decode.c	94.71% <0.00%> (+0.81%)	⬆️

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