Config: display user-friendly message when invalid generator is passed #1513

Job	Run time
Build Phar on PHP: 5.4 / Build Phar on PHP: 5.4	33s
Build Phar on PHP: 5.5 / Build Phar on PHP: 5.5	36s
Build Phar on PHP: 5.6 / Build Phar on PHP: 5.6	28s
Build Phar on PHP: 7.0 / Build Phar on PHP: 7.0	19s
Build Phar on PHP: 7.1 / Build Phar on PHP: 7.1	20s
Build Phar on PHP: 7.2 / Build Phar on PHP: 7.2	21s
Build Phar on PHP: 7.3 / Build Phar on PHP: 7.3	19s
Build Phar on PHP: 7.4 / Build Phar on PHP: 7.4	19s
Build Phar on PHP: 8.1 / Build Phar on PHP: 8.1	24s
Build Phar on PHP: 8.2 / Build Phar on PHP: 8.2	21s
Build Phar on PHP: 8.3 / Build Phar on PHP: 8.3	19s
Build Phar on PHP: 8.4 / Build Phar on PHP: 8.4	18s
Build Phar on PHP: nightly / Build Phar on PHP: nightly	18s
	4m 55s

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