Fix error handling

[ofek]

Motivation

      Traceback (most recent call last):
        File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/base.py", line 556, in run
          self.check(instance)
        File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/go_expvar/go_expvar.py", line 128, in check
          self.parse_expvar_data(data, tags, metrics, max_metrics, namespace)
        File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/go_expvar/go_expvar.py", line 168, in parse_expvar_data
          float(value)
      TypeError: float() argument must be a string or a number

[codecov]

Codecov Report

Merging #4505 into master will increase coverage by 15.01%.
The diff coverage is 0%.

Impacted Files	Coverage Δ
go_expvar/datadog_checks/go_expvar/go_expvar.py	82.3% <0%> (ø)
exchange_server/tests/test_exchange_server.py
datadog_checks_dev/tests/test_subprocess.py
disk/datadog_checks/disk/__init__.py
...checks_dev/tests/plugin/test_environment_runner.py
datadog_checks_dev/datadog_checks/dev/warn.py
datadog_checks_dev/tests/common.py
disk/tests/common.py
datadog_checks_dev/datadog_checks/dev/terraform.py
iis/datadog_checks/iis/__about__.py
... and 72 more