merge techs: throw error if not result file

[blond]

Error: ENOENT, no such file or directory '/Users/blond/projects/bem-components/desktop.examples/control-group/3A73mzkLO1r7wRGPTMAnGaOMXBo/3A73mzkLO1r7wRGPTMAnGaOMXBo.bemdecl.js'
    at Object.fs.statSync (fs.js:696:18)
    at Function.getFileInfo (/Users/blond/projects/bem-components/node_modules/enb/lib/file-list.js:189:19)
    at /Users/blond/projects/bem-components/node_modules/enb-bem-techs/techs/merge-bemdecl.js:99:33
    at Array.map (native)
    at module.exports.inherit.build (/Users/blond/projects/bem-components/node_modules/enb-bem-techs/techs/merge-bemdecl.js:98:46)
    at /Users/blond/projects/bem-components/node_modules/enb/lib/node.js:537:53
    at Array.map (native)
    at /Users/blond/projects/bem-components/node_modules/enb/lib/node.js:526:36
    at Array.1 (/Users/blond/projects/bem-components/node_modules/enb/node_modules/vow/lib/vow.js:202:56)
    at Object.callFns [as _onImmediate] (/Users/blond/projects/bem-components/node_modules/enb/node_modules/vow/lib/vow.js:462:35)