Raise meaningful error in reconstruct w bad method

If a method other than 'erosion' or 'dilation' was passed, an
`UnboundLocalVariable` error for `pad_value` was raised, which made it
difficult to determine the source of the error. Now, an eloquent
`ValueError` is raised instead.

See #1218.
Test correct error for wrong method in reconstruct
Merge pull request #1219 from jni/reconstruct-error

Raise meaningful error when wrong "method" passed to `reconstruct`