Revert "getteximage: Return correct error value when texure object is not found"

Revert "getteximage: Return correct error value when texure object is not found"

From OpenGL 4.5 spec PDF, section '8.11. Texture Queries', page 236:
  "An INVALID_VALUE error is generated if texture is not the name of
   an existing texture object."

Same wording applies to the compressed version.

But turns out this is a spec bug, and Khronos is fixing it for the next
revisions.

The proposal is to return INVALID_OPERATION in these cases.

This reverts commit 633c959faeae5099fd095f27da7b954e4a36254b.

v2:
- Use _mesa_lookup_texture_err (Samuel Pitoiset)

v3:
- _mesa_lookup_texture_err() already handles texture > 0 (Samuel
Pitoiset)
- Just revert 633c959fae (Juan A. Suarez)

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>