Add failOnError option to play nice with watch

[hurrymaplelad]

Make development time less frustrating. I like to do a full build before I setup watchers in development.

Currently, If I have a stray syntax error in a stylus file when I fire up my development environment grunt dies on the full build before the watchers are setup. I go fix the error, then have to manually start the development task again.

With this change, grunt still logs the stylus syntax error to help me troubleshoot, but happily carries on to setup watchers and rebuild styles as soon as I fix the error.

Inspired by grunt-shell and others.

Fixes #51. Alternative to #52.

[sindresorhus]

https://github.com/gruntjs/grunt-contrib-watch#optionsforever ?

[hurrymaplelad]

Watch's forever only kicks in after the watch has started. I've got Gruntfiles where I version image files, then build stylus files, then watch for changes in stylus sources. The first build can fail before watch clobbers fatal :(

[shama]

Actually would you mind just updating this to grunt.log.warn instead? Currently some tasks abort and other don't. I spoke to @cowboy about it awhile back and he said just warn on errors. See: gruntjs/grunt-contrib-csslint#15 (comment)

[hurrymaplelad]

@shama sounds good, I'll switch to warn.

[hurrymaplelad]

@shama done, but after making the change I don't like it. I feel pretty strongly that errors in tasks should cause grunt to exit with an error code to play nice in build scripts.

[vladikoff]

gruntjs/grunt#1163