fail build if toolchain not available

[pwj58]

When specifying a toolchain via --toolchains fail the build if the requested toolchain is not available.

Eg. Attempting to use gcc14, logs warning and error but then continues to build with the system default.

cuppa --toolchains=gcc14 --dbg --force-test --parallel --offline --develop
scons: Reading SConscript files ...
cuppa: configure: [info] No settings to load, skipping configure
cuppa: version: [info] cuppa: version 0.9.78
cuppa: construct: [info] Running in OFFLINE mode
cuppa: construct: [info] using sconstruct file [sconstruct]
cuppa: construct: [info] available toolchains are ['gcc92', 'gcc111', 'gcc103', 'gcc10', 'gcc']
cuppa: construct: [warn] Requested toolchain [gcc14] does not match any supported, skipping
cuppa: construct: [error] None of the requested toolchains are available

Eg. gcc7 is supported by cuppa but not installed, again builds with system default.

cuppa --toolchains=gcc7 --dbg --force-test --parallel --offline --develop
scons: Reading SConscript files ...
cuppa: configure: [info] No settings to load, skipping configure
cuppa: version: [info] cuppa: version 0.9.78
cuppa: construct: [info] Running in OFFLINE mode
cuppa: construct: [info] using sconstruct file [sconstruct]
cuppa: construct: [info] available toolchains are ['gcc92', 'gcc111', 'gcc103', 'gcc10', 'gcc']
cuppa: construct: [warn] Requested toolchain [gcc7] does not match any available, skipping
cuppa: construct: [error] None of the requested toolchains are available
cuppa: construct: [info] default profiles are ['']