BUG: Fix tests when FreeImage is not installed.

The plugin loader tries plugins but only catches ImportError and
RuntimeError.  The FreeImage plugin was throwing OSError.  Tests
were failing when FreeImage was not installed.  It now throws a
RuntimeError.

skimage/io/_plugins/freeimage_plugin.py

@@ -72,12 +72,12 @@ def load_freeimage():
             # No freeimage library loaded, and load-errors reported for some
             # candidate libs
             err_txt = ['%s:\n%s' % (l, str(e.message)) for l, e in errors]
-            raise OSError('One or more FreeImage libraries were found, but '
+            raise RuntimeError('One or more FreeImage libraries were found, but '
                           'could not be loaded due to the following errors:\n'
                           '\n\n'.join(err_txt))
         else:
             # No errors, because no potential libraries found at all!
-            raise OSError('Could not find a FreeImage library in any of:\n' +
+            raise RuntimeError('Could not find a FreeImage library in any of:\n' +
                           '\n'.join(lib_dirs))
 
     # FreeImage found

skimage/io/tests/test_freeimage.py

@@ -11,7 +11,7 @@ try:
     import skimage.io._plugins.freeimage_plugin as fi
     FI_available = True
     sio.use_plugin('freeimage')
-except OSError:
+except RuntimeError:
     FI_available = False
 
 
@@ -23,7 +23,7 @@ def setup_module(self):
     """
     try:
         sio.use_plugin('freeimage')
-    except OSError:
+    except RuntimeError:
         pass
 
 

skimage/io/tests/test_plugin.py

@@ -15,7 +15,7 @@ try:
     io.use_plugin('freeimage')
     FI_available = True
     priority_plugin = 'freeimage'
-except OSError:
+except RuntimeError:
     FI_available = False