bootmanager: Address review comments

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.