st/mesa: improve error messages and fix security warning

st/mesa: improve error messages and fix security warning

Debian, Ubuntu set default build flag: -Werror=format-security

  CC       state_tracker/st_cb_texturebarrier.lo
state_tracker/st_cb_eglimage.c: In function ‘st_egl_image_get_surface’:
state_tracker/st_cb_eglimage.c:64:7: error: format not a string literal and no format arguments [-Werror=format-security]
       _mesa_error(ctx, GL_INVALID_VALUE, error);
       ^~~~~~~~~~~
state_tracker/st_cb_eglimage.c:71:7: error: format not a string literal and no format arguments [-Werror=format-security]
       _mesa_error(ctx, GL_INVALID_OPERATION, error);
       ^~~~~~~~~~~

Reported-by: Krzysztof Kolasa <kkolasa@winsoft.pl>
Fixes: 83e9de25f325 ("st/mesa: EGLImageTarget* error handling")