Make the failure case of ToolkitRegistry.call more useful

[mattwthompson]

Right now, ToolkitRegistry.call tries the requested method in each of its wrapped toolkits and if they all fail, it raises a ValueError that squishes together the messages in each exception:

https://github.com/openforcefield/openff-toolkit/blob/7a3fad5239bcdb940fec8b28a4d4d27235f0738e/openff/toolkit/utils/toolkit_registry.py#L376-L380

A small improvement would involve raising a custom exception instead of ValueError, using mostly the same message.

cc: @j-wags who can provide more thoughts on other solutions and possibly if it's worth refactoring the design in general.