ucalls: use replace error handler (#1888)

Prevents the following error when tracing a java program that contains
non-ascii method name:

Traceback (most recent call last):
  File "/usr/share/bcc/tools/lib/ucalls", line 305, in <module>
    data = get_data()   # [(function, (num calls, latency in ns))]
  File "/usr/share/bcc/tools/lib/ucalls", line 266, in get_data
    bpf["counts"].items()))
  File "/usr/share/bcc/tools/lib/ucalls", line 264, in <lambda>
    kv[0].method.decode(),
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc4 in position 11: ordinal not in range(128)

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>