Added support of optional ignoring of errors to function calls

[ziflex]

This feature introduces a new capability to suppress/ignore errors caused by an invoked function.

For instance, the following query will always return a result even if a called function returns an error:

LET found = ELEMENT(doc, "#submit")?

RETURN found != NONE

New ? symbol at the end of any function call indicates that error must be suppressed and None value returned instead.

[codecov]

Codecov Report

Merging #652 (3fc8c92) into master (0cb7623) will increase coverage by 0.0%.
The diff coverage is 56.0%.

@@          Coverage Diff           @@
##           master    #652   +/-   ##
======================================
  Coverage    35.4%   35.4%           
======================================
  Files         335     335           
  Lines       12230   12249   +19     
======================================
+ Hits         4326    4338   +12     
- Misses       7486    7493    +7     
  Partials      418     418