provide more context to errors (#92)

* reverse the order of checks in the DimensionalityError constructor

* simplify the str method

* add a custom exception for invalid units

* use the new exception to present all invalid units at once

* replace the custom class by a formatter

* use the new formatter to collect and present unit errors

* fix a leftover reference to the custom error class

* synchronize DataArray quantify with Dataset quantify

* more removing of the custom error classes

* fix a bug in reindex and rely on convert_units to raise an error

* fix most other methods

* update whats-new.rst

* fix a link [skip-ci]

* explicitly parse the units

* limit the errors caught in quantify to ValueError and UndefinedUnitError

* remove the custom error class

* actually call loc when expecting errors

* also check whether the error for a invalid attribute is properly formatted

* rewrite the tests for to

docs/whats-new.rst

@@ -8,9 +8,9 @@ What's new
 - rewrite :py:meth:`Dataset.pint.quantify` and :py:meth:`DataArray.pint.quantify`,
   to use pint's `parse_units` instead of `parse_expression` (:pull:`40`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
-- ensure the variable which causes the error is explicit if an error occurs in
-  :py:meth:`Dataset.pint.quantify` (:pull:`43`)
-  By `Tom Nicholas <https://github.com/TomNicholas>`_.
+- ensure the variables which causes the error is explicit if an error occurs in
+  :py:meth:`Dataset.pint.quantify` and other methods (:pull:`43`, :pull:`91`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_ and `Justus Magin <https://github.com/keewis>`_.
 - refactor the internal conversion functions (:pull:`56`)
   By `Justus Magin <https://github.com/keewis>`_.
 - allow converting indexes (except :py:class:`pandas.MultiIndex`) (:pull:`56`)
@@ -45,7 +45,7 @@ What's new
   By `Justus Magin <https://github.com/keewis>`_.
 - implement :py:meth:`Dataset.pint.interpolate_na` and :py:meth:`DataArray.pint.interpolate_na` (:pull:`82`).
   By `Justus Magin <https://github.com/keewis>`_.
-- expose :py:func:`pint_xarray.accessors.setup_registry` as public API (:pull:`89`)
+- expose :py:func:`pint_xarray.setup_registry` as public API (:pull:`89`)
   By `Justus Magin <https://github.com/keewis>`_.
 
 v0.1 (October 26 2020)