mesa: fix error codes in _mesa_shader_source(), _mesa_get_shader_source()

If the 'shader' parameter is wrong, need to either generate GL_INVALID_VALUE
or GL_INVALID_OPERATION.  It depends on whether 'shader' actually names a
'program' or is a totally unknown ID.
There might be other cases to fix...

cherry-picked from master

src/mesa/shader/shader_api.c

 {
    struct gl_shader *sh = _mesa_lookup_shader(ctx, shader);
    if (!sh) {
-      _mesa_error(ctx, GL_INVALID_VALUE, "glGetShaderSource(shader)");
+      GLenum err;
+      if (_mesa_lookup_shader_program(ctx, shader))
+         err = GL_INVALID_OPERATION;
+      else
+         err = GL_INVALID_VALUE;
+      _mesa_error(ctx, err, "glGetShaderSource(shader)");
       return;
    }
    copy_string(sourceOut, maxLength, length, sh->Source);
 {
    struct gl_shader *sh = _mesa_lookup_shader(ctx, shader);
    if (!sh) {
-      _mesa_error(ctx, GL_INVALID_VALUE, "glShaderSource(shaderObj)");
+      GLenum err;
+      if (_mesa_lookup_shader_program(ctx, shader))
+         err = GL_INVALID_OPERATION;
+      else
+         err = GL_INVALID_VALUE;
+      _mesa_error(ctx, err, "glShaderSource(shaderObj)");
       return;
    }