[SIGMON-127 + SIGMON-98] Turn stacktraces into warnings

Context

• SIGMON-127 https://its.cern.ch/jira/browse/SIGMON-127
• SIGMON-98 https://its.cern.ch/jira/browse/SIGMON-98
• Experts reported that in case of the garbage signal (QDS from the second board (A/B)) the certain cells of the AN_RB_FPA.ipynb notebook craches.

What was done

• QdsAnalysis#analyze_qds_trigger returns empty result when the input signal is different than expected
• NqpsQuery#quesry_voltage_nqps returns empty DataFrame when the input signal is missing
• QdsAnalysis#plot_u_diode_nxcals warns about missing u_nqps data and uses NaN instead
• fixes to support non-existing timestamp_iqps in QdsAnalysis ([[]] -> reindex, at -> get)

Results

TODO

• mp3 experts will judge :)

QA

• sonar issues: http://sonar.cern.ch/project/issues?branch=SIGMON-127_turn_stacktrace_into_warning&id=lhc-sm-api&resolved=false
• some unit tests are written but the coverage is very low

Review objectives

• assessment of code functionality
• assessment of code quality
• identifying potential refactorings / logic common to other projects
• knowledge sharing