- New generic exception class TerraSyncPyException.
- Add subclass NetworkError of TerraSyncPyException.
- Raise a NetworkError exception when the HTTP return code is not 200.
- hash_of_file() does not silently ignore errors anymore; exceptions
should be dealt with wherever appropriate by the callers.
Whenever hash_of_file() returns, its return value is now the SHA-1
hash of the specified file. This is less error-prone IMHO than
returning None. Otherwise, calling code could erroneously conclude
that there is a matching hash when the file to check is actually
missing. For a concrete example, see the 'dirIndexHash' parameter of
TerraSync.updateDirectory(), which so far is used precisely with the
value None to express that "we are just starting the recursion and
have no hash from the server to compare to".
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