Panic thrown with invalid Log level

[ayh20]

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[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
Incorrect log level throws panic

Expected behavior:
Error messaging saying that the log level is invalid and saying what would be valid :-)

Minimal steps to reproduce the problem (not required if feature enhancement):
Only tested on Win under powershell...

Set env var ..... in my case i typed a zero rather than an O
$env:FLOGO_LOG_LEVEL = "INF0"

Run you app as normal ... ie
.\UDPTest.exe
Messages are:
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x20 pc=0x58182d]

goroutine 1 [running]:
UDPTest/vendor/github.com/TIBCOSoftware/flogo-lib/core/mapper/exprmapper/expression/function.Registry(0x8c10e0, 0xaf4c98)
D:/Data/Documents/Flogo/UDPTest/src/UDPTest/vendor/github.com/TIBCOSoftware/flogo-lib/core/mapper/exprmapper/expression/function/registry.go:33 +0x17d
UDPTest/vendor/github.com/TIBCOSoftware/flogo-contrib/action/flow/linker.init.0()
D:/Data/Documents/Flogo/UDPTest/src/UDPTest/vendor/github.com/TIBCOSoftware/flogo-contrib/action/flow/linker/isdefined.go:11 +0x44

What is the motivation / use case for changing the behavior?

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):
Windows 10, Powershell
Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X
0.5.2
Additional information you deem important (e.g. issue happens only occasionally):