Create a generic exception PayutcException

Remove also the error mapping since it is not used. Now all
exceptions will be serialized the same way, and if this is a
PayutcException then the httpcode will be 400, otherwise 500.
Fix ReadOnlyDatabaseTest class

Ensure the insertion is performed only one time
Merge branch 'master' into exceptions
Merge pull request #148 from trecouvr/fix-rodbtest

Fix ReadOnlyDatabaseTest class
Merge pull request #146 from trecouvr/exceptions

Ok travis a foiré (mais que sur une des deux versions de PHP et c'est encore l'échec de merde, de l'api github...) donc bref il est passé.