Avoid uncaught fatal errors in REST API if vocabulary ID not found

[osma]

Fixes #1170

This is a similar fix as #1215, but for the REST API.

For an URL like this: http://localhost/Skosmos/rest/v1/koko2/data?format=text/turtle

(note that there is no vocabulary with the id koko2 defined in config.ttl)

it prevents uncaught fatal errors like this:

[Tue Nov 02 09:49:37.020140 2021] [php7:error] [pid 175327] [client 127.0.0.1:53116] PHP Fatal error:  Uncaught ValueError: Vocabulary id 'koko2' not found in configuration. in /var/www/html/Skosmos/model/Model.php:420\nStack trace:\n#0 /var/www/html/Skosmos/model/Request.php(246): Model->getVocabulary()\n#1 /var/www/html/Skosmos/rest.php(46): Request->setVocab()\n#2 {main}\n  thrown in /var/www/html/Skosmos/model/Model.php on line 420

Instead a 404 error is produced, like this:

404 Not Found : Vocabulary id 'koko2' not found.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #1231 (09fa432) into master (5cf55f0) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1231   +/-   ##
=========================================
  Coverage     68.36%   68.36%           
  Complexity     1619     1619           
=========================================
  Files            32       32           
  Lines          3973     3973           
=========================================
  Hits           2716     2716           
  Misses         1257     1257           

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