Error out when the given duration is negative

[el10savio]

Fixes: #154

Validate processInfo period & log a fatal error in the case of negative duration.

Now gops exits and returns a fatal error in the case when the user supplies a negative duration to gops <pid> <duration> instead of defaulting to 0 as shown below.

Previously:

$ gops 21794 -1.3s
parent PID:     21703
threads:        7
memory usage:   0.040%
cpu usage:      0.000%
username:       admin
cmd+args:       /tmp/go-build3005809368/b001/exe/main
elapsed time:   15:39
local/remote:   127.0.0.1:38793 <-> 0.0.0.0:0 (LISTEN)

Now:

$ go build && ./gops 21794 -1s  
2021/11/10 18:18:26 Cannot process with negative duration: -1s

Note:

In an attempt to validate the duration values, I skipped over validating floats as internally they get converted to the appropriate duration. for example 0.5s internally gets converted to 500ms

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[el10savio]

@googlebot I signed it!

[tklauser]

Thanks for the PR! One minor nit regarding the log message wording, otherwise LGTM.