uhm, not so fast here 
I found a way to get a more detailed error log from the graphic driver. If you are curious, look here -> http://www.mesa3d.org/envvars.html
I did the following:
export LIBGL_DEBUG=verbose
export MESA_DEBUG=1
export MESA_LOG_FILE=/home/tuxator/mesa_errs.log
steam &>/home/tuxator/steam_errs.log
That resulted in some interesting errors that look like something in your game could be wrong:
Mesa: User error: GL_INVALID_VALUE in glViewport(0, 0, -1, -1)
Mesa: User error: GL_INVALID_ENUM in glMatrixMode(mode)
Mesa: User error: GL_INVALID_ENUM in glMatrixMode(mode)
Mesa: User error: GL_INVALID_VALUE in glViewport(0, 0, -1, -1)
Mesa: User error: GL_INVALID_ENUM in glMatrixMode(mode)
Mesa: User error: GL_INVALID_ENUM in glMatrixMode(mode)
What the guys are saying is that they can’t give you a definite answer and fix it for you, since you’re using an unstable release Ubuntu Linux. When they (Star Gems, Inc.) fix something now for your platform, it is no guarantee it will work later as the final public version is released, which can have significant differences in X11’s server, OpenGL stacks via mesa and of course the kernel may have code that is deemed incompatible with the current build of Star Conflict.
What they’re getting at is they don’t want to chase their own tails for a beta release, which makes sense. With that being said, I am a Linux and BSD developer and I work on the bleeding edge X11 server, Open GL stacks and other areas. I tell people that want to play this game, stay in 12.04.5, 14.04.2 or 14.10.1, don’t upgrade to 15.04 and expect things to work because more than likely it won’t.
My question to you is. Why did you download 15.04 and attempt to play Steam based games on the unstable beta platform? The thing about it is, the X11 server is not 100% complete on 15.04, no is the support in mesa 3-D drivers. The only people that should be downloading and installing 15.04 are developers and people that know enough about Ubuntu and have the balls to figure out the problems on their own, since there is no official support as of yet.
Besides, Ubuntu is trying to kill off the X11 server in favor of their Mir compositor, they wanted to introduce a meta version of that in 15.04 and be the beginning point for it, however, it’s not going well for them. They’re divided into three camps; pure X11, pure Mir and hybrid Mir interfacing to X11 to get business done. This is one of the reasons why 15.04 development is taking so long, they’re weighing all their options.
If you’re going to upgrade,the next non beta version of the LTS is 16.04 but wait for the next patched release, 16.04.1, where the majority of the bugs and security fixes have been made, then download and install it. Just something to think about.
The best version of Ubuntu that supports the restricted drivers of ATI and Nvidia is going to be 14.10. Ubuntu goofed on 12.04.5 and 14.04.2 because there is a bunch of .deb package dependency errors. The best is 14.10 because all you have to do is build to the X server version instead of the distribution, not all the time building .debs is the best option, especially when the package managers and associated dependencies are at odds with each other.