Parsing of bad values (syntactically and semantically) should be more strict

[azeey]

Example of bad syntax: #228
Example of bad semantics: RobotLocomotion/drake#12863 (review)

This is actually a spec change; "bad" values should throw an error by design, not be ignored by design. Currently, a negative effort limit implies ignoring a joint limit...

For handling bad semantics, look into using min and max values in the spec descriptions

For non-scalar values like //joint/axis/xyz, libsdformat should enforce that the axis is a unit vector by either re-normalizing or requiring it to be normalized.

Also, spec should say that it will disregard non-unit vectors for axes.

We should do PR for libsdformat10 as an error.

If we have extra time, backport to libsdformat9, but as a warning. (With updates to Gazebo Classic + Ignition Gazebo.)

[azeey]

Addressed by #244 and #259

[EricCousineau-TRI]

Marking as done, but spun off separate issue about //joint/axis/xyz
#307