Changes QMessageBox usages to warnings, as the problems they bring to
light are being safely handled by the application and do not warrant
something of the "critical" level.

Changes LOG_CRITICAL to LOG_ERROR for the same reason. Preferring ERROR
to WARNING as yuzu is denying loading of any guest applications after
checking for these conditions.

Moved logging the GL_RENDERER string into GetUnsupportedGLExtensions()
to make more clear that unsupported extensions were already being
logged. Makes placement of the logs easier to understand later, as well.