Fix failing tests

Author: saxbophone

basest/core/encode.py

@@ -4,6 +4,7 @@
     absolute_import, division, print_function, unicode_literals
 )
 
+from ..exceptions import ImproperUsageError
 from .utils import ints_to_symbols, symbols_to_ints, validate_symbol_tables
 
 
@@ -38,10 +39,10 @@ def encode_raw(input_base, output_base, input_ratio, output_ratio, input_data):
     Special validation: if the output base is larger than the input base, then
     the length of the input data MUST be an exact multiple of the input ratio.
     Otherwise, the data will be corrupted if we continue, so we will raise
-    ValueError instead.
+    ImproperUsageError instead.
     '''
     if input_base < output_base and input_length % input_ratio != 0:
-        raise ValueError(
+        raise ImproperUsageError(
             'Input data length must be exact multiple of input ratio when '
             'output base is larger than input base'
         )