internal/report: error if filter argument has no matches

[prattmic]

list, weblist, disasm, and peek all take a regexp argument that acts as
a filter.

Currently if this filter results in no matches we simply generate an
empty report, which can be confusing to users. Instead, explicitly treat
this as an error condition that is clearly reported to users.

Fixes #629

[prattmic]

I took the same approach with weblist as the rest of the commands, but I'm not totally confident in this. Since weblist has HTML output, it might be cleaner to include the error in the output HTML rather than failing generation entirely?

[codecov-commenter]

Codecov Report

Merging #631 (fe317d6) into master (a478d1d) will increase coverage by 0.60%.
The diff coverage is n/a.

❗ Current head fe317d6 differs from pull request most recent head ba2b8a6. Consider uploading reports for the commit ba2b8a6 to get more accurate results

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
+ Coverage   67.29%   67.89%   +0.60%     
==========================================
  Files          40       40              
  Lines        7310     7320      +10     
==========================================
+ Hits         4919     4970      +51     
+ Misses       1952     1909      -43     
- Partials      439      441       +2     

Impacted Files	Coverage Δ
.../github.com/google/pprof/internal/report/source.go	80.42% <0.00%> (+0.93%)	⬆️
.../github.com/google/pprof/internal/report/report.go	28.54% <0.00%> (+6.57%)	⬆️

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[prattmic]

PTAL