Allow passing in a column to plot for error_bars

The [`pygmt.fig.plot`](https://www.pygmt.org/v0.2.1/api/generated/pygmt.Figure.plot.html) method has an error_bar (E) argument that isn't very usable right now. Ideally we would be able to pass in a list or numpy array of error values (e.g. standard deviation).

> As agreed above, we'll figure out how to pass in an 'error' column with `error_bars (E)` and rename 'sizes' to 'size' in a separate PR next time.

_Originally posted by @weiji14 in https://github.com/GenericMappingTools/pygmt/issues/666#issuecomment-716084019_

There's also a Python package called `uncertainties` at https://github.com/lebigot/uncertainties/ which wraps error values around numpy arrays, it might be cool if we can integrate that with `pygmt` somehow! I.e. pass in an `uncertainty` array to `fig.plot()` and have it plot the error bars directly. Also could tie this in with the `pint` package at https://github.com/hgrecco/pint which has a concept of "Measurements" that includes the error with numbers.