ShaderGroup command parsing tweaks

1. Make testrender accept the new syntax as the contents of the ShaderGroup node, change all testrender tests to use it.

2. Make testshade exit with error code (rather than segfault!) if the group fails to be created.

3. ShaderGroupBegin: Better error message for group command parsing errors -- say the group name and try to hint which statement is the problem.

Example (note the badly formed 'foo' parameter that omits a type):

    $ testshade --group "int[5] iarray 1 2 4 5 6; foo 4 ; shader test;"
    ERROR: ShaderGroupBegin: error parsing group description: Unknown statement (expected 'param', 'shader', or 'connect'): "foo"
        group: "<unknown>"
        problem might be here:  foo 4 ;
    ERROR: Invalid shader group. Exiting testshade.