Fix get_identifiers_from_url() return codes

[GlazerMann]

The get_identifiers_from_url() return code is ONLY checked when adding a new URL in add_if_new().

Current code returns TRUE if the URL was converted into something else.
Proposed code also returns TRUE if the URL could have been converted, but the item already existed.
As it is code could add handle url for example even if hdl= was already set.

[codecov]

Codecov Report

Merging #517 into master will decrease coverage by 2.95%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master     #517      +/-   ##
============================================
- Coverage     75.73%   72.77%   -2.96%     
- Complexity     1433     1449      +16     
============================================
  Files             9        9              
  Lines          2464     2490      +26     
============================================
- Hits           1866     1812      -54     
- Misses          598      678      +80

Impacted Files	Coverage Δ	Complexity Δ
Template.php	79.65% <50%> (-4.22%)	1297 <0> (+16)
expandFns.php	95.16% <0%> (+0.11%)	0% <0%> (ø)	⬇️

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